Remove tui_default_win_viewport_height

Remove tui_default_win_viewport_height

tui_default_win_viewport_height was only called from a single spot,
for a single type of window.  This patch removes the function and
moves the logic into the sole caller.

gdb/ChangeLog
2019-10-09  Tom Tromey  <tom@tromey.com>

	* tui/tui-disasm.c (tui_get_low_disassembly_address): Compute
	window height directly.
	* tui/tui-layout.h (tui_default_win_viewport_height): Don't
	declare.
	* tui/tui-layout.c (tui_default_win_height): Remove.
	(tui_default_win_viewport_height): Remove.