Only delete in subfields by matching codes instead of codes and values

[dersmon]

Previously, if a field value was equal to the provided parameter, the value got treated as a code and the following (potentially non-existing) code as a value.

I just realised this is a duplicate of pull request https://github.com/edsu/pymarc/pull/91/commits/26a640058f5e68f9c2164e7f7ee0b4636c95d92e.

[edsu]

Thanks!