Allow erasing vars via function-scope

This triggered an assert because the remove code had no idea how to
find the function scope.

Oops!

src/env.cpp

@@ -1251,6 +1251,13 @@ mod_result_t env_stack_impl_t::remove(const wcstring &key, int mode) {
             result.global_modified = true;
         } else if (query.local) {
             result.status = remove_from_chain(locals_, key) ? ENV_OK : ENV_NOT_FOUND;
+        } else if (query.function) {
+            auto node = locals_;
+            while (node->next) {
+                node = node->next;
+                if (node->new_scope) break;
+            }
+            result.status = remove_from_chain(node, key) ? ENV_OK : ENV_NOT_FOUND;
         } else {
             DIE("Unknown scope");
         }

tests/checks/set.fish

@@ -806,3 +806,17 @@ end
 test-function-scope
 echo $funcvar $funcvar2
 # CHECK:
+
+function erase-funcvar
+    set -l banana 1
+    begin
+        set -l banana 2
+        set -ef banana
+        echo $banana
+        # CHECK: 2
+    end
+    echo $banana
+    # CHECK:
+end
+
+erase-funcvar