Bread and butter pickles without mustard seed
Mustard seeds and other spices are there for flavor only; it's perfectly safe to leave them out. But don't mess with amounts of salt or vinegar given in your recipe--those are important for preventing bacterial growth.

If your recipe also contains bay leaf or grape leaf, those too can be omitted, but you pickles won't be as crisp. Likewise if you omit the calcium chloride or calcium hydroxide.