Remove dependencies check

This is a check for whether the server has successfully implemented the
extension. This is both a rare occurence (haven't seen this in practice
yet) and/or something we should just ignore anyway.

We already do the important client side check - implied extensions -
elsewhere.
Better capability remove() method

Need to be able to remove individual extension parameters, since, e.g.,
an end-user may want to disable 1 of many parameters via the
capability_ignore parameter.