Bug 1290531 - Remove support for building with custom build.sh; r=dustin

Now that tester-device is gone, there are no more images using custom
build.sh scripts and that feature can be deleted. Yay simplicity.

MozReview-Commit-ID: 5JqFZ2HOsWs

--HG--
extra : rebase_source : 437bc8e877be065bf85db81422f3c1ddd5ac2800

testing/docker/build.sh

@@ -11,14 +11,9 @@ build() {
   local tag=$2
   local folder="$gecko_root/testing/docker/$image_name"
 
-  if [ -f $folder/build.sh ]; then
-    shift
-    $folder/build.sh -t $tag $* || exit 1
-  else
   # use --no-cache so that we always get the latest updates from yum
   # and use the latest version of system-setup.sh
   ( cd $folder/.. && docker build --no-cache -t $tag $image_name ) || exit 1
-  fi
 
   echo "Success built $image_name and tagged with $tag"
 }