Removed activation logic for OP_REVERSEBYTES to pretend it was always enabled

Summary:
Activation logic is now unnecessary, assuming this code doesn't travel back in time.
This change removes the scriptflag SCRIPT_ENABLE_OP_REVERSEBYTES and all the places it gets activated.
Also entirely removes the activation tests.

Test Plan:
ninja check
ninja check-functional

Reviewers: deadalnix, O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6173