Remove tui_gen_win_info::viewport_height

Remove tui_gen_win_info::viewport_height

tui_gen_win_info::viewport_height is only used in a couple of spots,
and is redundant with "height".  This patch removes viewport_height.

gdb/ChangeLog
2019-12-27  Tom Tromey  <tom@tromey.com>

	* tui/tui-source.c (tui_source_window::maybe_update): Update.
	* tui/tui-regs.c (tui_data_window::display_registers_from):
	Update.
	* tui/tui-layout.c (tui_gen_win_info::resize): Update.
	* tui/tui-data.h (struct tui_gen_win_info) <viewport_height>:
	Remove.
	* tui/tui-command.c (tui_cmd_window::resize): Update.

Change-Id: I020e026fbe289adda8e2fdfebca91bdbdbc312e8