MAJOR: acl: remove fetch argument validation from the ACL struct

ACL fetch being inherited from the sample fetch keyword, we don't need
anymore to specify what function to use to validate the fetch arguments.

Note that the job is still done in the ACL parsing code based on elements
from the sample fetch structs.
diff --git a/src/acl.c b/src/acl.c
index a996c63..2e23e30 100644
--- a/src/acl.c
+++ b/src/acl.c
@@ -1052,7 +1052,7 @@
 			if (!expr->args)
 				expr->args = empty_arg_list;
 
-			if (aclkw->val_args && !aclkw->val_args(expr->args, err)) {
+			if (aclkw->smp->val_args && !aclkw->smp->val_args(expr->args, err)) {
 				/* invalid keyword argument, error must have been
 				 * set by val_args().
 				 */