package: Drop support for PACKAGEBUILDPKGD function customisation

We don't really need the customisation capability of PACKAGEBUILDPKGD
since we also have PACKAGE_PREPROCESS_FUNCS which is generally used
by recipes. Drop PACKAGEBUILDPKGD to have some simpler/clearer code.
This allows some function wrappers to be removed too.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>