Omit IF_LINT code that no longer seems needed

Nowadays GCC is smarter, or the Emacs code has mutated, or both,
and now is as good a time as any to remove uses of IF_LINT that
now seem to be unnecessary.
* lib-src/emacsclient.c (set_local_socket):
* lib-src/movemail.c (main) [MAIL_USE_MAILLOCK && HAVE_TOUCHLOCK]:
* src/buffer.c (fix_start_end_in_overlays, fix_overlays_before):
* src/casefiddle.c (casify_region):
* src/charset.c (load_charset_map):
* src/coding.c (decode_coding_object, encode_coding_object):
* src/data.c (Fmake_variable_buffer_local, Fmake_local_variable)
(cons_to_unsigned, cons_to_signed):
* src/frame.c (make_frame, x_set_frame_parameters):
* src/keyboard.c (read_event_from_main_queue):
* src/regex.c (regex_compile):
* src/syntax.c (back_comment):
* src/window.c (Frecenter):
* src/xfaces.c (Fx_list_fonts):
Remove IF_LINT that no longer seems necessary.
* src/image.c (png_load_body, jpeg_load_body): Simplify use of IF_LINT.
* src/keyboard.c (read_char): Use IF_LINT (volatile) rather than
a pragma dance to pacify GCC -Wclobbered.
* src/xdisp.c (x_produce_glyphs): Rewrite to avoid need for IF_LINT.

* src/xterm.c (x_connection_closed): Now _Noreturn, which should
mean we do not need IF_LINT any more.
(x_io_error_quitter): Now _Noreturn.  Put an 'assume (false)’
at the end, to forestall warnings from older compilers.

lib-src/emacsclient.c

@@ -1195,7 +1195,7 @@ set_local_socket (const char *local_socket_name)
     int use_tmpdir = 0;
     int saved_errno;
     const char *server_name = local_socket_name;
-    const char *tmpdir IF_LINT ( = NULL);
+    const char *tmpdir;
     char *tmpdir_storage = NULL;
     char *socket_name_storage = NULL;