Fix C-x 1 from gdb prompt

Fix C-x 1 from gdb prompt

Pedro pointed out on irc that C-x 1 from the gdb prompt will cause a
crash.  This happened because of a bug in remove_windows -- it would
always remove all the windows from the layout.  This patch fixes this
bug, and also arranges to have C-x 1 preserve the status window.

gdb/ChangeLog
2020-06-16  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.c (tui_layout_split::remove_windows): Fix logic.
	Also preserve the status window.