[tests] Remove bip9-softforks.py

bip9-sofforks.py was intended to be a generic test for versionbits
deployments. However, it only tests CSV activation and was not updated
to test segwit activation. CSV activation is tested by
bip68-112-113-p2p.py, so this test is duplicated effort.

Rather than try to update it to use the BitcoinTestFramework, just
remove it.

Author: jnewbery

test/functional/feature_bip9_softforks.py

@@ -159,7 +159,6 @@
     'mining_getblocktemplate_longpoll.py',
     'p2p_timeouts.py',
     # vv Tests less than 60s vv
-    'feature_bip9_softforks.py',
     'p2p_feefilter.py',
     'rpc_bind.py',
     # vv Tests less than 30s vv