[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/bip9-softforks.py

@@ -157,7 +157,6 @@
     'getblocktemplate_longpoll.py',  # FIXME: "socket.error: [Errno 54] Connection reset by peer" on my Mac, same as  https://github.com/bitcoin/bitcoin/issues/6651
     'p2p-timeouts.py',
     # vv Tests less than 60s vv
-    'bip9-softforks.py',
     'rpcbind_test.py',
     # vv Tests less than 30s vv
     'assumevalid.py',