Remove useless calls to setexcessiveblock in extended tests

Summary:
The excessive block size is already set via the -excessiveblocksize
option, so the calls to the setexcessiveblock RPC is redundant.
This causes the tests to fail since D8918.

Test Plan:

./test/functional/test_runner.py \
  abc_p2p_compactblocks \
  abc_p2p_fullblocktest

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8926