tests: remove obsolete code to create custom build directories

We now do in fact do this and decide this.

Change-Id: I6a6a696493b288ed76d809122bda259dc0225846

tests/functional/check.sh

@@ -37,9 +37,6 @@ if checkBuildTempDirRemoved $TEST_ROOT/log; then false; fi
 test_custom_build_dir() {
   local customBuildDir="$TEST_ROOT/custom-build-dir"
 
-  # Nix does not create the parent directories, and perhaps it shouldn't try to
-  # decide the permissions of build-dir.
-  mkdir "$customBuildDir"
   nix-build check.nix -A failed --argstr checkBuildId $checkBuildId \
       --no-out-link --keep-failed --option build-dir "$TEST_ROOT/custom-build-dir" 2> $TEST_ROOT/log || status=$?
   [ "$status" = "100" ]