Remove some dead code from tui_set_layout

Remove some dead code from tui_set_layout

tui_set_layout sets regs_populate using:

      regs_populate = (new_layout == SRC_DATA_COMMAND
		       || new_layout == DISASSEM_DATA_COMMAND);

Then later it checks this variable:

	  if (!regs_populate
	      && (new_layout == SRC_DATA_COMMAND
		  || new_layout == DISASSEM_DATA_COMMAND))

However, this is equivalent to "!regs_populate && regs_populate",
which can never be true.  So, remove the dead code and the variable.

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

	* tui/tui-layout.c (tui_set_layout): Remove regs_populate
	variable.