tests: hard_hole_fits and T16875 are duplicated

ghc(master)$ find . -name 'T16875.hs'        
./testsuite/tests/perf/compiler/T16875.hs
./testsuite/tests/typecheck/should_compile/T16875.hs
ghc(master)$ find . -name 'hard_hole_fits.hs'
./testsuite/tests/perf/compiler/hard_hole_fits.hs
./testsuite/tests/typecheck/should_compile/hard_hole_fits.hs

From the git log, I see the following:

1. @mpickering moved those files to the perf/ subtree in 11c9a469
2. @AndreasK reintroduced them in typecheck/ in 680ef2c8 (bad rebase?)

We should probably remove those from the typecheck/ subtree, along with the .stderr files.