DTrace tst.gcc.d isn't useful
the DTrace tests contain a test, tst.gcc.d, which is the same as tst.coverage.d, except built using GCC.
This made sense when Studio was the primary compiler, and likely to remain so, but currently leaves it duplicate.
Further, in a multi-shadow world (#9128) there's no realistic way to say "Build this test with every other compiler". We should just remove it.