exceptions.c:print_flush: Remove obsolete check

Commit 20f0d60d ("Avoid crash when calling warning too early"),
added a "current_top_target () != NULL" check to
target_supports_terminal_ours, so this check in exceptions.c is now
obsolete.

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

	* exceptions.c (print_flush): Remove current_top_target() check.