Migrate to Android Testing Support Lib (part 6/N)

This is a follow up CL to previous CLs that converted unit tests to
use Android Testing Support Library but forgot to remove unused import
lines.

Bug: 110805255
Test: verified as follows. No new test failures.
        tapas adb LatinIME LatinIMETests arm64 userdebug && \
        DISABLE_PROGUARD=true make -j LatinIME && \
        adb install -r $OUT/system/app/LatinIME/LatinIME.apk && \
        atest LatinIMETests:com.android.inputmethod.latin
Change-Id: I8762b0dfc209bdf68867f725b03a22286026faae