Remove redundant OLD_FILES entries for debug symbols.

Entries for foo.debug files matching an existing entry in OLD_FILES or
OLD_LIBS are unnecessary as they are auto-generated.

Reviewed by: imp, emaste
Sponsored by: The University of Cambridge, Google Inc.
Differential Revision: https://reviews.freebsd.org/D33777

(cherry picked from commit ebc1c246955d798d51749ec7d51147385b71a3f1)