refactor: Remove unused COINBASE_FLAGS

Commit d449772cf6 stopped setting
COINBASE_FLAGS, and it looks like it hasn't been used since P2SH.
Update the help string to remove "flags", which is not specified in
BIP 22.

src/validation.cpp

@@ -129,9 +129,6 @@ CFeeRate minRelayTxFee = CFeeRate(DEFAULT_MIN_RELAY_TX_FEE);
 CBlockPolicyEstimator feeEstimator;
 CTxMemPool mempool(&feeEstimator);
 
-/** Constant stuff for coinbase transactions we create: */
-CScript COINBASE_FLAGS;
-
 // Internal stuff
 namespace {
     CBlockIndex* pindexBestInvalid = nullptr;