avcodec: Remove a misplaced and useless attribute_deprecated

If attribute_deprecated is used in an enum declaration, it
should follow the 'enum' keyword, otherwise it's ignored
silently. This is the only case of attribute_deprecated for
enum declarations currently.

Currently, this attribute_deprecated doesn't have any effect.
If moved to the right place, it emits a warning every single
time avcodec.h is included, like this:

avcodec.h:2827: warning: ‘AVLPCType’ is deprecated (declared at avcodec.h:543)

There is already a working attribute_deprecated for the
corresponding field in AVCodecContext, so therefore this
one shouldn't be needed.

Signed-off-by: Martin Storsjö <martin@martin.st>