reverting "jenkins: remove build history directory before collecting srcrevs"

This reverts commit af6f1b4c4e.

This revert shall test if this is the reason that the nightly
build takes that long...

Jenkinsfile

@@ -304,7 +304,7 @@ def getTopUpstreamBuildNumber() {
 }
 
 def getAutoRevHashes() {
-  sh(script: "bash -c '. ./env.image-ostree && rm -rf buildhistory && bitbake -f netmodule-linux-image-dev --runall=fetch'")
+  sh(script: "bash -c '. ./env.image-ostree && bitbake netmodule-linux-image-dev --runall=fetch'")
   def revs = sh(returnStdout: true, script: "bash -c \". ./env.image-ostree > /dev/null && buildhistory-collect-srcrevs\"").trim()
   return revs
 }