Remove unused codepaths that do deltas from resin/scratch

The supervisor has been doing regular pulls instead of deltas
from scratch for a while now. We remove remaining references to
resin/scratch, and add a handler in docker-utils to fall back
to a regular pull with a null deltaSource (which should never be
called anyways, but is left as a precaution).

Change-type: patch
Signed-off-by: Pablo Carranza Velez <pablo@balena.io>

src/application-manager.coffee

@@ -793,7 +793,7 @@ module.exports = class ApplicationManager extends EventEmitter
 		for availableImage in available
 			if availableImage.appId == image.appId
 				return availableImage.name
-		return 'resin/scratch'
+		return null
 
 	# returns:
 	# imagesToRemove: images that

src/compose/images.coffee

@@ -59,7 +59,7 @@ module.exports = class Images extends EventEmitter
 			.catch =>
 				@reportChange(image.imageId, _.merge(_.clone(image), { status: 'Downloading', downloadProgress: 0 }))
 				Promise.try =>
-					if validation.checkTruthy(opts.delta) and opts.deltaSource? and opts.deltaSource != 'resin/scratch'
+					if validation.checkTruthy(opts.delta) and opts.deltaSource?
 						@logger.logSystemEvent(logTypes.downloadImageDelta, { image })
 						@inspectByName(opts.deltaSource)
 						.then (srcImage) =>

src/lib/docker-utils.coffee

@@ -59,14 +59,13 @@ module.exports = class DockerUtils extends DockerToolbelt
 		{
 			deltaRequestTimeout, deltaApplyTimeout, deltaRetryCount, deltaRetryInterval,
 			uuid, currentApiKey, deltaEndpoint, apiEndpoint,
-			deltaSource, deltaSourceId, deltaVersion, startFromEmpty = false
+			deltaSource, deltaSourceId, deltaVersion
 		} = fullDeltaOpts
 		retryCount = checkInt(deltaRetryCount)
 		retryInterval = checkInt(deltaRetryInterval)
 		requestTimeout = checkInt(deltaRequestTimeout)
 		applyTimeout = checkInt(deltaApplyTimeout)
 		version = checkInt(deltaVersion)
-		deltaSource = 'resin/scratch' if startFromEmpty or !deltaSource?
 		deltaSourceId ?= deltaSource
 
 		log = (str) ->
@@ -76,6 +75,12 @@ module.exports = class DockerUtils extends DockerToolbelt
 			log("Unsupported delta version: #{version}. Falling back to regular pull")
 			return @fetchImageWithProgress(imgDest, fullDeltaOpts, onProgress)
 
+		# Since the supervisor never calls this function without a source anymore,
+		# this should never happen, but we handle it anyways.
+		if !deltaSource?
+			log("Falling back to regular pull due to lack of a delta source")
+			return @fetchImageWithProgress(imgDest, fullDeltaOpts, onProgress)
+
 		docker = this
 
 		log("Starting delta to #{imgDest}")
@@ -110,7 +115,7 @@ module.exports = class DockerUtils extends DockerToolbelt
 							if not (300 <= res.statusCode < 400 and res.headers['location']?)
 								throw new Error("Got #{res.statusCode} when requesting image from delta server.")
 							deltaUrl = res.headers['location']
-							if deltaSource is 'resin/scratch'
+							if !deltaSource?
 								deltaSrc = null
 							else
 								deltaSrc = deltaSourceId