Delete test validation

Error prone has a check for missing @Test, so we don't need it.

Now that we have Ravenizer that can see all test code at the build time,
we no longer need these checks, at least at runtime. If needed, we can
now detect them at the build time.
Test: build
Bug: 361656170
Flag: EXEMPT host test change only

Change-Id: Ib4b94c3e49b507fefd931bc4bf46b11f4424ed02