API Rescue Master Branch PR: Remove isDev / isTest querystring arguments

[GuySartorelli]

Rescues 215d93c
Refers to https://www.silverstripe.org/download/security-releases/SS-2018-005

Parent issue

• Rescue master branch commits #10350

[emteknetnz]

There was a bunch of removed unit tests in the original commit - do we need to worry about those?

[GuySartorelli]

Those tests have actually changed since then to explicitly check that you can't use the isDev and isTest get parameters to change the env type - which is exactly what this rescued commit is intending to do (by outright removing that functionality). That's also why the CI is green despite not removing those tests.
It could be argued that the tests aren't necessary now that the functionality is removed, but because this is related to a security issue I decided it was best to keep them.
Happy to remove if you think they're not needed anymore.