[signingscript] remove duplicate vendored/mozbuild

[escapewindow]

At the top level of the repo, we have a vendored/mozbuild from mozilla-central rev 462fc9264901, which iscript uses.

We also have a signingscript/src/signingscript/vendored/mozbuild from mozilla-central rev 462fc9264901, which signingscript uses.

I'm hoping we can remove the signingscript copy, and point to the shared copy. We'll need to update signingscript's setup.py and Dockerfile accordingly.