cm: Remove ahbottomnavigation library

We aren't currently using this. The only reason why this was
being synced, was because it's a dependency of the rebased
Gallery app.

Since the Gallery rebase is currently stalled, remove the library
for now.

This essentially reverts commit 58a7613.

Change-Id: I029979484ec3c695aebbe90eb2a32fc23aaa24f1

snippets/cm.xml

@@ -3,7 +3,6 @@
 
   <!-- LineageOS additions -->
   <project path="android" name="LineageOS/android" />
-  <project path="external/ahbottomnavigation" name="LineageOS/android_external_ahbottomnavigation" />
   <project path="external/android-visualizer" name="LineageOS/android_external_android-visualizer" />
   <project path="external/bash" name="LineageOS/android_external_bash" />
   <project path="external/cmsdk-api-coverage" name="LineageOS/android_external_cmsdk-api-coverage" />