Remove gdb_usleep.c
    
    I noticed that gdb_usleep is unused, so this patch removes it.
    
    gdb/ChangeLog
    2019-09-26  Tom Tromey  <tom@tromey.com>
    
    	* Makefile.in (COMMON_SFILES): Remove gdb_usleep.c.
    	(HFILES_NO_SRCDIR): Remove gdb_usleep.h.
    	* gdb_usleep.h: Remove.
    	* gdb_usleep.c: Remove.
    	* utils.c: Don't include gdb_usleep.h.