Updated with RemoveAllModifiersFromSource()

Item.cs

public class Item
{
    public void Equip(Character c)
    {
        // Create the modifiers and set the Source to "this"
        // Note that we don't need to store the modifiers in variables anymore
        c.Strength.AddModifier(new StatModifier(10, StatModType.Flat, this));
        c.Strength.AddModifier(new StatModifier(0.1, StatModType.Percent, this));
    }

    public void Unequip(Character c)
    {
        // Remove all modifiers applied by "this" Item
        c.Strength.RemoveAllModifiersFromSource(this);
    }
}