remove _comma_separated

Just realised you can do the same thing with _values

src/_comma_separated

@@ -1,11 +0,0 @@
-#autoload
-
-# This function can be used to complete comma separated values.
-# The first argument should be a description for the completion items,
-# and any further args are the completion items themselves.
-
-compset -P '*,'
-local -a used vals
-used=(${(s:,:)IPREFIX})
-_wanted tty expl $1 compadd -S ',' -q -F used "$@[2,-1]"
-