Remove <imi> entries with zero <subtype> from subtype.xml

As mentioned in its JavaDoc, the way to remove additional subtypes
from the IME is passing an empty InputMethodSubtype array as folows:

  imm#setAdditionalInputMethodSubtypes(imi, new InputMethodSubtype[]);

While there is no problem from the viewpoint of IME developers, a
harmless but redundant entry for that IME remains in subtype.xml.

  <subtypes>
     <imi id="com.android.inputmethod.latin/.LatinIME" />
  </subtypes>

With this CL, such a redundant entry will be removed.

Bug: 121223050
Test: Manually verified as follows
  1. Build and flash aosp_taimen-userdebug into taimen
  2. Make sure that the device fully boots up
  3. adb reboot  # to avoid Bug 121259290
  4. adb root
  5. adb shell cat /data/system/inputmethod/subtypes.xml
      -> make sure the content looks as follows:
         <subtypes>
           <imi id="com.android.inputmethod.latin/.LatinIME">
             <subtype ....>
             <subtype ....>
           </imi/>
         </subtypes>
  6. Open AOSP Keyboard settings
  7. Go to "Appearance & Layouts" -> "Custom input styles"
  8. Remove all layouts.
  9. adb shell cat /data/system/inputmethod/subtypes.xml
      -> make sure the content looks as follows:
         <subtypes>
         </subtypes>
Change-Id: I30820e570e42d5dbc4fd1b6e311b2a8d2553be6d