Remove reset_locator

reset_locator was introduced just a few patches ago, but it's already
time to remove it.  It consists of a call to the locator's "reset"
method, plus a call to tui_make_window; but the latter is redundant at
all the places that call reset_locator.

gdb/ChangeLog
2019-07-17  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.c (show_source_disasm_command, show_data):
	Update.
	(reset_locator): Remove.
	(show_source_or_disasm_and_command): Update.