Remove some main thread assertions that are not helping

This is to make experimenting with concurrent execution easier.
No functional change in this commit.

src/builtin_source.cpp

@@ -23,7 +23,6 @@
 /// The  source builtin, sometimes called `.`. Evaluates the contents of a file in the current
 /// context.
 maybe_t<int> builtin_source(parser_t &parser, io_streams_t &streams, const wchar_t **argv) {
-    ASSERT_IS_MAIN_THREAD();
     const wchar_t *cmd = argv[0];
     int argc = builtin_count_args(argv);
     help_only_cmd_opts_t opts;

src/complete.cpp

@@ -479,7 +479,6 @@ bool completer_t::condition_test(const wcstring &condition) {
         return false;
     }
 
-    ASSERT_IS_MAIN_THREAD();
     bool test_res;
     auto cached_entry = condition_cache.find(condition);
     if (cached_entry == condition_cache.end()) {

src/exec.cpp

@@ -181,7 +181,6 @@ bool is_thompson_shell_script(const char *path) {
 /// This function is similar to launch_process, except it is not called after a fork (i.e. it only
 /// calls exec) and therefore it can allocate memory.
 [[noreturn]] static void launch_process_nofork(env_stack_t &vars, process_t *p) {
-    ASSERT_IS_MAIN_THREAD();
     ASSERT_IS_NOT_FORKED_CHILD();
 
     // Construct argv. Ensure the strings stay alive for the duration of this function.

src/input.cpp

@@ -857,7 +857,6 @@ static std::vector<terminfo_mapping_t> create_input_terminfo() {
 }
 
 bool input_terminfo_get_sequence(const wcstring &name, wcstring *out_seq) {
-    ASSERT_IS_MAIN_THREAD();
     assert(s_input_initialized);
     for (const terminfo_mapping_t &m : *s_terminfo_mappings) {
         if (name == m.name) {

src/io.cpp

@@ -76,7 +76,6 @@ ssize_t io_buffer_t::read_once(int fd, acquired_lock<separated_buffer_t> &buffer
 }
 
 void io_buffer_t::begin_filling(autoclose_fd_t fd) {
-    ASSERT_IS_MAIN_THREAD();
     assert(!fillthread_running() && "Already have a fillthread");
 
     // We want to fill buffer_ by reading from fd. fd is the read end of a pipe; the write end is

src/iothread.cpp

@@ -253,7 +253,6 @@ int thread_pool_t::perform(void_function_t &&func, bool cant_wait) {
 }
 
 void iothread_perform_impl(void_function_t &&func, bool cant_wait) {
-    ASSERT_IS_MAIN_THREAD();
     ASSERT_IS_NOT_FORKED_CHILD();
     s_io_thread_pool.perform(std::move(func), cant_wait);
 }

src/parser.cpp

@@ -417,9 +417,6 @@ wcstring parser_t::stack_trace() const {
 /// is of type FUNCTION_CALL. If the caller doesn't specify a starting position in the stack we
 /// begin with the current block.
 const wchar_t *parser_t::is_function(size_t idx) const {
-    // PCA: Have to make this a string somehow.
-    ASSERT_IS_MAIN_THREAD();
-
     for (size_t block_idx = idx; block_idx < block_list.size(); block_idx++) {
         const block_t &b = block_list[block_idx];
         if (b.is_function_call()) {
@@ -476,8 +473,6 @@ int parser_t::get_lineno() const {
 }
 
 const wchar_t *parser_t::current_filename() const {
-    ASSERT_IS_MAIN_THREAD();
-
     for (const auto &b : block_list) {
         if (b.is_function_call()) {
             return function_get_definition_file(b.function_name);

src/postfork.cpp

@@ -168,8 +168,6 @@ int child_setup_process(pid_t new_termowner, pid_t fish_pgrp, const job_t &job,
 /// FORK_LAPS times, with a very slight delay between each lap. If fork fails even then, the process
 /// will exit with an error message.
 pid_t execute_fork() {
-    ASSERT_IS_MAIN_THREAD();
-
     if (JOIN_THREADS_BEFORE_FORK) {
         // Make sure we have no outstanding threads before we fork. This is a pretty sketchy thing
         // to do here, both because exec.cpp shouldn't have to know about iothreads, and because the