Remove unused InputMethodSettings#getEnabledInputMethodNames()

This is a follow up CL to my previous CL [1], with which we stopped
using

  InputMethodSettings#getEnabledInputMethodNames()

but we forgot to remove it.

With this CL, this unused method is actually removed.

 [1]: Iac98fdbb2758f4d9c68930f49d219eb83e54a3d0
      6c984cca

Bug: 261723412
Bug: 309837937
Test: presubmit
Change-Id: Ibf8429efb31ecc4ec18448fad1e809f28a715fa2