Stop generating `has` and `clear` for repeated extension fields

[thomasvl]

Currently a has and clear method is generated for each property we create for extension fields. But for repeated fields, that's sorta pointless. It makes more sense to treat them like normal message fields, and just let folks use isEmpty and assignment to the empty list for these.

While they should strip away if not used, the problem is the file is generated with visibility public then the code likely comes along and lives in the final framework or binary (SR-521, SR-1021).

To transition to this, we could generated the current ones, but tag them with Swift's @available to tag the current ones as deprecated and then eventually drop them.