Revert "InputManagerService: Do not remove input channel when disposing monitor"

This reverts commit b3c30218.

The bug in Looper was fixed in AOSP, so we can now undo the workaround.

Bug: 195020232
Bug: 189135695
Test: None
Change-Id: Icc831d9bf63b0695899f163b70d1ccfd81b772be