Remove unused parameter from two TUI functions

Remove unused parameter from two TUI functions

The "display_prompt" parameter of tui_erase_source_content and
tui_clear_source_content was never passed the NO_EMPTY_SOURCE_PROMPT
value, so remove the parameter.  Once this is done, the
EMPTY_SOURCE_PROMPT and NO_EMPTY_SOURCE_PROMPT defines are unused, so
remove those as well.

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

	* tui/tui-winsource.h (tui_erase_source_content)
	(tui_clear_source_content): Remove "display_prompt" parameter.
	* tui/tui-winsource.c (tui_update_source_window_as_is)
	(tui_update_source_windows_with_addr): Update.
	(tui_clear_source_content): Remove "display_prompt" parameter.
	(tui_erase_source_content): Likewise.  Simplify.
	(tui_show_source_content): Update.
	* tui/tui-win.c (tui_resize_all, tui_adjust_win_heights): Update.
	* tui/tui-stack.c (tui_show_frame_info): Update.
	* tui/tui-data.h (EMPTY_SOURCE_PROMPT, NO_EMPTY_SOURCE_PROMPT):
	Remove defines.