Remove some more ASSERT_IS_MAIN_THREADs

These aren't helping and are blocking testing of concurrent execution.
No functional change here.

src/builtin_complete.cpp

@@ -124,8 +124,6 @@ static void builtin_complete_print(const wcstring &cmd, io_streams_t &streams, p
 /// The complete builtin. Used for specifying programmable tab-completions. Calls the functions in
 // complete.cpp for any heavy lifting.
 maybe_t<int> builtin_complete(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);
     completion_mode_t result_mode{};

src/io.cpp

@@ -136,7 +136,6 @@ void io_buffer_t::begin_filling(autoclose_fd_t fd) {
 
 separated_buffer_t io_buffer_t::complete_background_fillthread_and_take_buffer() {
     // Mark that our fillthread is done, then wake it up.
-    ASSERT_IS_MAIN_THREAD();
     assert(fillthread_running() && "Should have a fillthread");
     assert(this->item_id_ > 0 && "Should have a valid item ID");
     shutdown_fillthread_ = true;