Remove external/r8

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

Bug: 65854021
Merged-In: I9c70b9d408b7208a94bd8ef0584c1aa6fb94abc6
Change-Id: I9c70b9d408b7208a94bd8ef0584c1aa6fb94abc6
1 file changed
tree: 542ab780f2c2f1131dd9f14897ce5a3a45099ed5
  1. GLOBAL-PREUPLOAD.cfg
  2. default.xml