Fix license enums

commit 97271e8c:

cerbero: Remove unused license enums
There's no corresponding license files for these, and we don't know of
any packages that actually use these. Every package uses the -Plus
variants.

With this, people will get an error message when the recipe is parsed
instead of getting a cryptic error later during install.

See: https://gitlab.freedesktop.org/gstreamer/cerbero/issues/143

commit 43a014e0:

cerbero: Improve error message when using BSD/MIT licenses

commit 444eba4d:

cerbero: Do not require a license file for proprietary licenses
We can assume in this case that whoever wrote the recipe is either not
redistributing the binaries, or will handle licensing externally.

Fixes https://gitlab.freedesktop.org/gstreamer/cerbero/issues/143