Remove translation from Spree::LineItem

Due to [I assume] the Rails load order, it this translation prohibits include/prepend/extend decoration of Spree::LineItem as the locales aren’t loaded yet. I also believe this to be redundant, as there is a before_validation method to ensure the quantity is never less than 0 on #L15

core/app/models/spree/line_item.rb

@@ -18,8 +18,7 @@ class LineItem < Spree::Base
  validates :variant, presence: true
  validates :quantity, numericality: {
  only_integer: true,
- greater_than: -1,
- message: Spree.t('validation.must_be_int')
+ greater_than: -1
  }
  validates :price, numericality: true
  validate :ensure_proper_currency