WIP: ldb: no tarballs any more

Merge request to track additional patch on top of !374 to remove the tarball. This isn't likely to go anywhere in the short-term until some more fundamental questions are sorted out however.

This removes the distinct ldb release process, instead ldb will be released with Samba.

This avoids confusion where every new symbol causes an LDB release to be tagged and a tarball prepared.

For now ldb can still be built from lib/ldb in the checkout or tarball, if desired.