Remove various empty or useless functions

In particular remove init()/destroy() functions that do nothing, or
destroy functions that only free memory.

src/fish_key_reader.cpp

@@ -299,7 +299,6 @@ static void setup_and_process_keys(bool continuous_mode) {
     restore_term_mode();
     restore_term_foreground_process_group();
     input_destroy();
-    reader_destroy();
 }
 
 static bool parse_debug_level_flag() {