Remove V6 compatibility

[joelanman]

Resolves #1295

It adds complexity to the codebase that we no longer need. Removing it makes other work we want to do like removing Gulp easier

To do:

• Update changelog (this will be a breaking change)
• Review tests, the update script test touches on v6 but I don't really understand why

[fofr]

Comparing this with #1123

I think you need to:

• remove lib/v6/govuk_template_unbranded.html
• remove unused dependencies from package.json (and the lock file)

[joelanman]

We may want to consider deprecating this for a period before removing it. This could warn users who use it (however they will have to update to see the warning). I think I'm more in favour of just removing, it's been a long time. Deprecating would add a month or so to this work to have a chance of being an effective warning.

[joelanman]

closing in favour of #1432