Remove IInputMethodManager#switchToNextInputMethod()

It turns out that we had already rejected null IME token in
InputMethodManager#switchToNextInputMethod() since Android L [1].

Hence there is no need to keep this IPC any more.

There should be no developer-visible behavior change.

 [1]: I043aa30a19c821f33effd57dfd6590b0e3ed817b
      34c666472137a99a2ce5546b80bd04979d10ab7a

Bug: 114488811
Test: atest CtsInputMethodServiceHostTestCases
Change-Id: I72ee82d62e3bdce44f623604eca86ab3fe3df0bd