gdb/dwarf: don't pass lowpc/highpc to dwarf2_ranges_read in cooked_indexer::scan_attributes

It's not necessary to pass pointers to those variables
(dwarf2_ranges_read checks then for nullptr), and they are not used.
Remove them.

Change-Id: Id5462d481f209e9546380f52ddeea2eea811f5f3
Approved-By: Tom de Vries <tdevries@suse.de>