Remove redundant assignment from dwarf2_find_containing_comp_unit

Remove redundant assignment from dwarf2_find_containing_comp_unit

dwarf2_find_containing_comp_unit has two assignments to "this_cu" in
quick succession, both of which are just:

  this_cu = dwarf2_per_objfile->all_comp_units[low];

... with no intervening assignments.

This patch removes the second assignment.  I'm checking this in as
obvious.  Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-03-11  Tom Tromey  <tromey@adacore.com>

	* dwarf2read.c (dwarf2_find_containing_comp_unit): Remove
	redundant assignment to "this_cu".