[7/n InputDispatcher refactor] refactor removeInputChannel

Refactor removeInputChannelLocked so that it can be moved out to a
separate class in upcoming CL.

Bug: 367661487
Bug: 245989146
Test: atest inputflinger_tests
Flag: EXEMPT refactor
Change-Id: Ieac763aa5ada504b4ee76ff6d9a872949cb49e14