Don't call erase_data_content from tui_data_window::show_registers

Don't call erase_data_content from tui_data_window::show_registers

tui_data_window::show_registers currently calls erase_data_content.
However, I think it's better to have fewer calls to this (ideally just
one would suffice).  This refactors that function to remove this call.

gdb/ChangeLog
2019-10-09  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.c (tui_data_window::show_registers): Don't call
	erase_data_content.