gio-tool: Remove unnecessary exit() call

All the calls to `usage()` are immediately followed by a `return` from
`main()` which sets an appropriate exit status.

Calling `exit()` early means that running `gio --help` returns exit
status 1, which is incorrect — it should (by convention) return 0.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

gio/gio-tool.c

@@ -247,7 +247,6 @@ usage (void)
   g_printerr ("  tree     %s\n", _("Lists the contents of locations in a tree"));
   g_printerr ("\n");
   g_printerr (_("Use %s to get detailed help.\n"), "“gio help COMMAND”");
-  exit (1);
 }
 
 int