ALSA: pcm: Remove unused functions declaration

Remove function declarations for functions that don't have a matching
implementation.

For snd_pcm_build_linear_format the implementation was removed in
64d27f96 ("[ALSA] Support 3-bytes 24bit format in PCM OSS
emulation"). All the others never had one (as far as git history goes).

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>