time_util_unittest: remove sleep test for monotonic_raw

Monotonic raw clocks do not promise anything w.r.t. the real time. This
makes checking whether |X| seconds has passed after sleeping |X| seconds
infeasible as a sanity check.

In this CL, we remove the offending test.

Bug: 289894845
Tag: #floss
Test: atest bluetooth_test_common:bluetooth_test_common64.TimeTest#test_time_get_os_monotonic_raw_us_increases_lower_bound -- --abi x86_64
Change-Id: I1fe9efd86f2a18f2178d55e2a757a6fa45f51f7f