Remove an unnecessary null check

This is a preparation before fixing Bug 27859687.

Previously

  MyPackageMonitor#onFinishPackageChangesInternal()

skipped several tasks when

  Settings.Secure.DEFAULT_INPUT_METHOD

is empty.

This check does not make much sense and should be removed.

Bug: 27859687
Bug: 322062773
Test: atest CtsInputMethodInstallTestCases:AdditionalSubtypeLifecycleTest
Change-Id: I07e32739f486d960c9dd22476120fa35bf1899e4