Remove May, 15 opcode activation code

Summary: It is clear by now that we won't get a reorg that deep (we even have a checkpoint) and they are strictly additive, so no need to keep activation code around.

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1481