sound: Remove feed_matrix_apply_generic()

It does what feed_matrix_apply() already does, so it is redundant.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48036

(cherry picked from commit b73b5f70e9f6be951e66531f6f57249eea4ba4dc)