gdb/dwarf: remove unnecessary per_objfile parameter in cooked_indexer::ensure_cu_exists

The per_objfile object can be obtained from the cutu_reader.  This is
actually how both callers get it in order to pass it as argument.

Change-Id: Iac134ded247d841f80ab5ca55dd9055b556410c3
Approved-By: Tom Tromey <tom@tromey.com>
2 files changed