Remove unused InputMethodManagerService#mLastSwitchUserId

A special shell command for CTS

  adb shell cmd input_method get-last-switch-user-id,

which was added as a temporary workaround for multi-user IME scenario
tests [1], has no longer been used after our migration from host-side
IME CTS tests to device-side tests [2].

Now we can safely remove this shell command.

 [1]: I07a0120d44bcb3e3d60d51c510e7e566ce424148
      a7babbbf
 [2]: Ic6ac9d440a95240973e6578459c82955de4d62df
      346afc790098c9c62decd4619c8681ae9f4de60e

Fix: 327495933
Test: atest CtsInputMethodServiceHostTestCases
Change-Id: Ia7d3cd218badc589566cdb04d719e523d5336ae4