Remove opencv3 from the manifest

Nothing is using it.
Bug: 34621082
Test: builds
Change-Id: Id958566059812a528bc90cad90d336f2687f54c8

default.xml

@@ -269,7 +269,6 @@
   <project path="external/objenesis" name="platform/external/objenesis" groups="pdk" />
   <project path="external/okhttp" name="platform/external/okhttp" groups="pdk" />
   <project path="external/opencv" name="platform/external/opencv" groups="pdk-cw-fs,pdk-fs" />
-  <project path="external/opencv3" name="platform/external/opencv3" groups="pdk" />
   <project path="external/owasp/sanitizer" name="platform/external/owasp/sanitizer" groups="pdk" />
   <project path="external/parameter-framework" name="platform/external/parameter-framework" groups="pdk" />
   <project path="external/pcre" name="platform/external/pcre" groups="pdk" />