fix(oauth2): only enable via GOOGLE_CLOUD_CPP_ENABLE

Enabling `oauth2` if `GOOGLE_CLOUD_CPPP_ENABLE_REST` is set is a bug: it
introduces a cycle in the dependencies of the library, and makes it
impossible to compile `oauth2`, `storage`, and `compute` in different
shards.

cmake/GoogleCloudCppFeatures.cmake

@@ -362,9 +362,6 @@ macro (google_cloud_cpp_enable_cleanup)
         OR (sql IN_LIST GOOGLE_CLOUD_CPP_ENABLE)
         OR (generator IN_LIST GOOGLE_CLOUD_CPP_ENABLE))
         set(GOOGLE_CLOUD_CPP_ENABLE_REST ON)
-        # Backwards compatibility. In the original release of `oauth2` we
-        # automatically compiled the library if REST was enabled
-        list(APPEND GOOGLE_CLOUD_CPP_ENABLE oauth2)
     endif ()
 
     list(REMOVE_DUPLICATES GOOGLE_CLOUD_CPP_ENABLE)