BigDecimal creation: Remove ArgumentError guard

`String#to_d` will never, ever raise an ArgumentError. I tried with
`Object.new.to_s.to_d`.

This raises our test coverage for these files.

This was originally introduced to guard against weird behaviour of Ruby
2.4, which we do not support any longer.

Author: mamhoff

core/lib/spree/preferences/preferable.rb

@@ -160,11 +160,7 @@ def convert_preference_value(value, type, preference_encryptor = nil)
         when :password
           value.to_s
         when :decimal
-          begin
-            value.to_s.to_d
-          rescue ArgumentError
-            Spree::ZERO
-          end
+          value.to_s.to_d
         when :integer
           value.to_i
         when :boolean

legacy_promotions/app/models/spree/calculator/tiered_flat_rate.rb

@@ -35,8 +35,6 @@ def compute(object)
 
     def cast_to_d(value)
       value.to_s.to_d
-    rescue ArgumentError
-      Spree::ZERO
     end
 
     def preferred_tiers_content

legacy_promotions/app/models/spree/calculator/tiered_percent.rb

@@ -42,8 +42,6 @@ def compute(object)
 
     def cast_to_d(value)
       value.to_s.to_d
-    rescue ArgumentError
-      Spree::ZERO
     end
 
     def preferred_tiers_content

promotions/app/models/solidus_promotions/calculators/tiered_flat_rate.rb

@@ -40,8 +40,6 @@ def compute_item(object)
 
       def cast_to_d(value)
         value.to_s.to_d
-      rescue ArgumentError
-        Spree::ZERO
       end
 
       def preferred_tiers_content

promotions/app/models/solidus_promotions/calculators/tiered_percent.rb

@@ -47,8 +47,6 @@ def compute_item(object)
 
       def cast_to_d(value)
         value.to_s.to_d
-      rescue ArgumentError
-        Spree::ZERO
       end
 
       def preferred_tiers_content