Simplify source window clearing

Simplify source window clearing

When a TUI source window is empty, it displays a "No Source Available"
message.  The function tui_set_source_content_nil also made sure to
put this message into the window's "content" field.

However, I believe this isn't really necessary.  Instead, it's simpler
to just empty the contents and let curses handle the refreshing.

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

	* tui/tui-winsource.c (tui_erase_source_content): Clear the
	window's contents.
	* tui/tui-source.h (tui_set_source_content_nil): Don't declare.
	* tui/tui-source.c (tui_set_source_content_nil): Remove.