chore(pack): stop tracking C2ME runtime cache

cache/c2me-dfc is regenerated on every server start (the .class files
even swap contents between runs) and is already excluded from the pack
by .packwizignore. Drop it from git and add a .gitignore so it cannot
be committed again.

.gitignore

@@ -0,0 +1,2 @@
+# C2ME runtime worldgen class cache (regenerated at runtime; see .packwizignore)
+cache/