[avalanche] Remove unused mock time from the quorum functional test

Summary: This is unused in the test, so no change in behavior.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_quorum

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11615