Remove an unused constructor of InputMethodSettings.

The last caller of the deprecated constructor of InputMethodSettings was
just removed [1].  Since it is not a public API, we can now safely
remove it.

  [1]: I09cba4066b95c4a9e89a3e4f83d75b97882502dc
       143a6869476a4be5962d4bba3b222d078f46b9a0

Bug: 26279466
Change-Id: I9035c417d6a8166c0ef4e4c7a00c151ffdd5fb49