Remove external/bison

This is now built in the build-tools branch on AOSP, and submitted to
the platform tree as prebuilts under prebuilts/build-tools. There's no
need for the source to exist in the platform tree anymore.
Test: m
Change-Id: Icbd169b34c51348a6ceb8b083405e9243ba13c89
Merged-In: Icbd169b34c51348a6ceb8b083405e9243ba13c89

default.xml

@@ -99,7 +99,6 @@
   <project path="external/autotest" name="platform/external/autotest" groups="pdk-fs" />
   <project path="external/avb" name="platform/external/avb" groups="pdk" />
   <project path="external/bart" name="platform/external/bart" groups="pdk" />
-  <project path="external/bison" name="platform/external/bison" groups="pdk" />
   <project path="external/blktrace" name="platform/external/blktrace" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/boringssl" name="platform/external/boringssl" groups="pdk" />
   <project path="external/bouncycastle" name="platform/external/bouncycastle" groups="pdk" />