Revert "Add platform/external/boringssl"

This reverts commit 5f5e0440bb.

default.xml

@@ -68,7 +68,6 @@
   <project path="external/bison" name="platform/external/bison" groups="pdk" />
   <project path="external/blktrace" name="platform/external/blktrace" groups="pdk-cw-fs" />
   <project path="external/bluetooth/bluedroid" name="platform/external/bluetooth/bluedroid" groups="pdk" />
-  <project path="external/boringssl" name="platform/external/boringssl" />
   <project path="external/bouncycastle" name="platform/external/bouncycastle" groups="pdk" />
   <project path="external/bsdiff" name="platform/external/bsdiff" groups="pdk" />
   <project path="external/bzip2" name="platform/external/bzip2" groups="pdk" />