remove unused function

The fish_key_reader program was the only user of the
`set_wait_on_escape_ms()` function and that use was removed with commit
0461743. So remove it from the main fish code. This was found by `make
lint`.

src/input_common.cpp

@@ -158,9 +158,6 @@ static wint_t readb() {
     return arr[0];
 }
 
-// Directly set the input timeout.
-void set_wait_on_escape_ms(int ms) { wait_on_escape_ms = ms; }
-
 // Update the wait_on_escape_ms value in response to the fish_escape_delay_ms user variable being
 // set.
 void update_wait_on_escape_ms() {

src/input_common.h

@@ -86,9 +86,6 @@ void input_common_destroy();
 /// Adjust the escape timeout.
 void update_wait_on_escape_ms();
 
-/// Set the escape timeout directly.
-void set_wait_on_escape_ms(int ms);
-
 /// Function used by input_readch to read bytes from stdin until enough bytes have been read to
 /// convert them to a wchar_t. Conversion is done using mbrtowc. If a character has previously been
 /// read and then 'unread' using \c input_common_unreadch, that character is returned. If timed is