Enable TETHER_FORCE_RANDOM_PREFIX_BASE_SELECTION by default on 25Q2+

Since the feature was fully rolled out via experiment without
regression, this commit removes the flag and the corresponding
logic in PrivateAddressCoordinator. The flag is no longer
necessary and simplifies the code.

Test: atest ConnectivityCoverageTests:PrivateAddressCoordinatorTest
Fix: 312647670
Change-Id: I8781f98b7e99ed0856e9e04112056df5adca2527
3 files changed