gdb/dwarf: remove unnecessary abfd parameter in dwarf2_per_bfd::locate_sections

The parameter `abfd` is always the same as `this->obfd`, there is no
need to pass it as a parameter.

Change-Id: If7ad58ad4efdf6b070cbf2b8a73436bd8b452fa6
Approved-By: Tom Tromey <tom@tromey.com>