Remove the -deprecatedrpc=getblocktemplate_sigops rpc option

Summary:
This was added for compatibility reason in v0.26.8 which is old enough to be safely removed.

Depends on D14369.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14370