clear_detail can only be called on TUI source windows

clear_detail can only be called on TUI source windows

The clear_detail method can only be called on source windows, so
remove definitions from the base of the class hierarchy, leaving only
a single non-virtual method.

gdb/ChangeLog
2019-08-13  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (struct tui_source_window_base)
	<clear_detail>: No longer "override".
	* tui/tui-regs.h (struct tui_data_window) <clear_detail>: Remove.
	* tui/tui-regs.c (tui_data_window::clear_detail): Remove.
	* tui/tui-data.h (struct tui_win_info) <clear_detail>: Remove.
	* tui/tui-command.h (struct tui_cmd_window) <clear_detail>:
	Remove.
	* tui/tui-command.c (tui_cmd_window::clear_detail): Remove.