Remove stray checkThread()

Looks like a copy/paste error during rebase in
40172abf

Also remove import of unused android.os.HandlerThread,
which was also added in same patch, and fixes push error:

 [COMMIT 159f07327053] Remove stray checkThread()
 [FAILED] checkstyle_hook
     Running Checkstyle on 159f07327053b2ea5db4ac7e550a3881ba75c883 commit
     ERRORS:
       [LineageOS/android_frameworks_opt_telephony] src/java/com/android/internal/telephony/IccPhoneBookInterfaceManager.java:23:8: Unused import: android.os.HandlerThread.

Change-Id: I2fd4503ede351d380ed5d8ec51e4a40af8eec0c6