sound: Remove unused code from pcm/g711.h

These routines are already implemented in pcm/intpcm.h.

No functional change intended.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D47734

(cherry picked from commit b882251204128388eb2f8e4f74e83ff1ca7863c4)