Remove primer test on black-primer (#5924)

black-primer was removed from black repository, so primer test on this
directory has been removed to prevent the CI from failing.

See psf/black#2924

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

tests/primer/packages_to_lint_batch_one.json

@@ -1,7 +1,7 @@
 {
   "black": {
     "branch": "main",
-    "directories": ["src/black/", "src/blackd/", "src/black_primer/", "src/blib2to3/"],
+    "directories": ["src/black/", "src/blackd/", "src/blib2to3/"],
     "url": "https://github.com/psf/black.git"
   },
   "django": {