Remove buildSrc (#2379)

We don't use the upload results feature (kokoro picks the results
artifacts directly and uploads them).

Keeping it around is a maintenance burden.

Also fixed a deprecation warning.

gradle.properties

@@ -13,9 +13,6 @@ org.gradle.parallel=true
 mavenUrl=
 pluginsUrl=
 allowInsecureProtocol=
-uploaderDestination=
-uploaderCredentialsFile=
-uploaderMultithreadedUpload=
 verboseTestOutput=false
 enableDependencyLocking=true
 enableCrossReferencing=false