Deprecate the setexcessiveblock RPC

Summary:
There already is a configuration option to set the excessive block size.
Removing this RPC fixes several potential bugs:

• It is potentially racey, as there is no lock on the parameter
• The CConman object hold its own reference, and will use the wrong value after this RPC is called.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8918