Remove unnecessary IMMS#calledFromValidUser()

If an IPC end point is exposed only to IMEs via
IInputMethodPrivilegedOperations.aidl, there is no need to call
calledFromValidUser() because the caller verification is expected to
be already done by calledWithValidTokenLocked().

Bug: 34886274
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: I5f124dd5c4dcabe5e0d6bce9056251c5a8a95321