tfile_target::close: trace_fd can't be -1
    
    It's not possible to open a tfile target with an invalid trace_fd, and
    it's not possible to close a closed target, so this early return is dead.
    
    gdb/ChangeLog:
    yyyy-mm-dd  Pedro Alves  <palves@redhat.com>
    
    	* tracefile-tfile.c (tfile_target::close): Assert that trace_fd is
    	not -1.