Remove body of tui_locator_window constructor

The tui_locator_window constructor initializes the first character of
two of its members.  However, this is actually an error, since these
were changed to be std::string.  This removes the erroneous code.

gdb/ChangeLog
2020-07-01  Tom Tromey  <tom@tromey.com>

	* tui/tui-stack.h (struct tui_locator_window): Remove body.