Now that the server team has switched to using enterprise-rhel-62-64-bit-dynamic-required as its main required build variant for running tasks, selected_tests_buildvariants no longer reflects the current list of required builders.

As a server engineer,
I want selected_tests.py to generate tasks on enterprise-rhel-62-64-bit-dynamic-required and the other required builders when I run it,
so that tasks are generated on the correct build variants.

AC:

• selected_tests_buildvariants are removed from etc/evergreen.yml: https://github.com/mongodb/mongo/blob/a121dd87e2c3b9268f173ed45d815f9f52ddc821/etc/evergreen.yml#L12738 and the required variants are identified some other way