Remove MetaHasTraits metaclass

[mdickinson]

Traits doesn't need the MetaHasTraits metaclass: everything it needs to be able to do it can do with the aid of __init_subclasses__, introduced in Python 3.6.

See #1551 for POC of PR that removes MetaHasTraits; the additional changes needed to make the test suite pass are fairly small. We'd also need to look for places elsewhere in ETS and beyond that MetaHasTraits is used. We'd also want to do some minimal profiling, just to check for major performance regressions.

Related: #1549.