Remove LineItem#normalize_quantity callback

This was mainly used for OrderContents#update_cart, but because the
method now deletes LineItems manually with quantities of 0 or less,
this is no longer needed.

core/app/models/spree/line_item.rb

@@ -22,7 +22,6 @@ class LineItem < Spree::Base
     has_many :line_item_actions, dependent: :destroy
     has_many :actions, through: :line_item_actions
 
-    before_validation :normalize_quantity
     before_validation :set_required_attributes
 
     validates :variant, presence: true
@@ -133,11 +132,6 @@ def pricing_options
 
     private
 
-    # Sets the quantity to zero if it is nil or less than zero.
-    def normalize_quantity
-      self.quantity = 0 if quantity.nil? || quantity < 0
-    end
-
     # Sets tax category, price-related attributes from
     # its variant if they are nil and a variant is present.
     def set_required_attributes