Drop building wheel for python 3.4 (#6406)

https://github.com/matthew-brett/multibuild/pull/240

kokoro/release/python/linux/build_artifacts.sh

@@ -50,7 +50,6 @@ build_artifact_version() {
 }
 
 build_artifact_version 2.7
-build_artifact_version 3.4
 build_artifact_version 3.5
 build_artifact_version 3.6
 build_artifact_version 3.7

kokoro/release/python/macos/build_artifacts.sh

@@ -51,7 +51,6 @@ build_artifact_version() {
 }
 
 build_artifact_version 2.7
-build_artifact_version 3.4
 build_artifact_version 3.5
 build_artifact_version 3.6
 build_artifact_version 3.7