Remove implicit parameter from GetSerializeSize

Summary:
Both parameters are integer so we can't rely on the type checker to raise errors.

One such error was present in GetVirtualTransactionSize

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3195