Simplify IsOpcodeDisabled

Summary:
Cyclomatic complexity goes from 27 to 17.

Also remove inline qualifier that is completely useless here.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1217