Remove cacheInput related function in Transaction.js

[matiu]

cacheInput related functions doesnt seem to be used anywhere, and they have no test. We should remove them to improve code quality and to not confuse new developers.

[matiu]

Note this:

#80

as evidence that it is confusing

[maraoz]

Agreed. Transaction.js needs a huge refactor. We might need separate files with helper classes (e.g. TransactionIn, TransactionOut, TransactionBuilder, maybe TransactionSigner)

[matiu]

Maybe we can start by removing legacy code, without breaking backwards compatibility.

[ryanxcharles]

Agreed