Check behavior when dropping a field that's part of the current partition spec

[Fokko]

Feature Request / Improvement

While reviewing this: https://github.com/apache/iceberg-python/pull/931/files#diff-23e8153e0fd497a9212215bd2067068f3b56fa071770c7ef326db3d3d03cee9bR467

I believe we should disallow dropping a field when it is part of the current partition spec. We should add checks for both the REST and non-REST catalog.