Remove unnecessary setting of defaults on xunit.net / nunit / (tunit?)

[nohwnd]

Work linked in #4307 (comment) tied the options to IsTestingPlatform and fixed setting the default values to true, so you no longer need to set them to true if you want default value of true.

Once MSTest 3.9.0 ships we can update the frameworks to the latest MTP and remove / revert the properties that set our default values.

Unless the framework wants to use a different value of course.

e.g. like this for NUnit

https://github.com/nunit/nunit3-vs-adapter/compare/master...nohwnd:revert-additional-customizations?expand=1