Remove not-required assertion in BluetoothTestUtils.disable/enable

Those assertions will cause LE builds failed as BluetoothAdapter behaves
a bit different on LE. And this removal should not have impacts on
exiting tests because Semaphore will guarantee BT is disabled/enabled
within timeout.

Bug: 111329335
Test: Do Bluetooth Enable Stress test on LE and RoW builds.
Change-Id: Ieedec90d21f4fd1aa3cdb4a6cb5849a9fa4fd864