Remove not-yet-implemented methods from CharacterData.

This speeds up a web-platform-tests run by about 45 seconds (of 11-12 minutes)
by avoiding three timeouts.
auto merge of #2184 : Ms2ger/servo/remove-nyi-characterdata, r=jdm

This speeds up a web-platform-tests run by about 45 seconds (of 11-12 minutes)
by avoiding three timeouts.