[functorch] Remove test/test_functorch_lagging_op_db.py (pytorch/functorch#845)

These tests are expected to fail, but we didn't communicate that very
well and:
1. we have gotten multiple questions about them
2. we need to special case it in our CI
3. we don't even use the test anymore!

So we are deleting it.

Related: pytorch/functorch#835

Author: zou3519