Simplify source and disassembly window creation

Simplify source and disassembly window creation

Similar to the previous patch, neither make_source_window nor
make_disasm_window could be called when *win_info_ptr was non-NULL.
This patch simplifies the functions by removing the parameter and
having them return their results directly.

gdb/ChangeLog
2019-06-25  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.c (make_source_window, make_disasm_window)
	(make_source_or_disasm_window): Remove win_info_ptr parameter.
	Return the new window.
	(show_source_disasm_command, show_data)
	(show_source_or_disasm_and_command): Update.