Revert "Add a flag dot_keep_unusable_xport_sec"

This reverts commit 9fc1b07c.

The test added in this change is flaky in R and blocks the train
release. This change also caused a bug that DnsTlsDispatcher::cleanup()
is called every DoT query if dot_keep_unusable_xport_sec is -1.

Bug: 233345976
Bug: 235965058
Test: atest
Change-Id: I884bc6ae611de474ca337eaa1174fef0da7215f5