Remove external/r8

We're now always using the jars from prebuilts/r8.

Bug: 65854021
Merged-In: I9c70b9d408b7208a94bd8ef0584c1aa6fb94abc6
Change-Id: I9c70b9d408b7208a94bd8ef0584c1aa6fb94abc6

default.xml

@@ -294,7 +294,6 @@
   <project path="external/protobuf" name="platform/external/protobuf" groups="pdk" />
   <project path="external/python/cpython2" name="platform/external/python/cpython2" groups="pdk" />
   <project path="external/python/cpython3" name="platform/external/python/cpython3" groups="pdk" />
-  <project path="external/r8" name="platform/external/r8" groups="pdk" />
   <project path="external/regex-re2" name="platform/external/regex-re2" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/replicaisland" name="platform/external/replicaisland" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/rmi4utils" name="platform/external/rmi4utils" />