Get rid of manager-release branch support in testing framework (SCT, DTest)

[mikliapko]

Currently, we are supporting two branches in two testing frameworks used for Manager testing (SCT, DTest):

• master;
• manager-release (manager-3.3 currently).

Since I started working with Manager test about half a year ago, we have never encountered a case where there were test-specific differences between these two branches. At the same time, manager-release branch support takes time on backporting and conflicts resolution.

To sum it up, I see no reason to support both of them and suggest to stay with only one master branch.

If there's a need for a separate release branch, we can create it any time.

[karol-kokoszka]

We are not really backporting fixes from master to branches other than the current release.
It makes sense to get rid of manager-release branch in SCT and Dtest.