Simplify dwarf2_find_containing_comp_unit

Simplify dwarf2_find_containing_comp_unit

In an earlier patch discussion we noticed that
dwarf2_find_containing_comp_unit takes the address of sect_off, but
doesn't actually need to.  This is a leftover from before
C++-ification.  This patch simplifies the function.

Tested using gdb.dwarf2 on x86-64 Fedora 28.

gdb/ChangeLog
2018-12-18  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (dwarf2_find_containing_comp_unit): Don't take
	address of sect_off.