This removes the unused == operator from CMutableTransaction.
The motivation is that unused code has a cost but offers no benefit (in general), while also adding the risk of introducing silent bugs. On top of that this particular code is quite inefficient, unnecessarily calculating the hash (it could, say, compare serializations). So if anyone ever needs to use a == comparison on CMutableTransaction, they'd be better of having to reimplement it (and add tests) than relying on code that's not being maintained.
Note: after this, trying to use the == operator on CMutableTransactions results in a compilation error:
./primitives/transaction.h:405:15: error: invalid operands to binary expression ('CMutableTransaction' and
'CMutableTransaction')
