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:
2020-01-10  Pedro Alves  <palves@redhat.com>

	* tracefile-tfile.c (tfile_target::close): Assert that trace_fd is
	not -1.