tests: remove flaky mocktime test

Summary:
This test is sometimes triggering an assertion on slow CI machines, because of an overflow. The issue can be reliably reproduced by adding a time.sleep(2) after this setmocktime call.

AssertionError: Unexpected stderr bitcoind: /home/pierre/dev/bitcoin-abc/src/util/time.cpp:78: static NodeClock::time_point NodeClock::now(): Assertion `ret > 0s' failed. !=

The maximum possible mocktime in seconds depends on each system, and is generally not max_int64. For instance on linux the time point is stored with nanoseconds precision, so the maximum would be max_int64 / 10^9.
See D15926 for an attempt to fix the issue (not trivial).

This diff just removes the flaky test and does not attempt to compute the actual allowable maximum that should be tested.

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15927