testsuite: Drop T7995
This test is quite sensitive to the build configuration as it requires that ghc
have unfoldings, which isn't true in the quick build flavours. I considered
various options to make the test more robust but none of them seemed
particularly appealing. Moreover, Simon PJ was a bit skeptical of the value of
the test to begin with and I strongly suspect that any regression in Trac #7995
would be accompanied by failures in our other compiler performance tests.
Closes Trac #17399.