Remove snapshot_blob.bin from dependency checks and docs

[amaitland]

CEF has stopped shipping the file (Chromium apparently did long ago).

https://bitbucket.org/chromiumembedded/cef/commits/719f423e7030540f9b334d12e2ef9f82fc759a36

We will need to update the dependency checker.

Currently it looks like this will land in M134

[amaitland]

Given that it's not officially required and it looks like Chromium stopped shipping In M66 we might be able to do this early as the file will still be included, we just won't check for it's existence.