Remove Fillable module (#271)

It is not necessary

spec/form_spec.cr

@@ -179,7 +179,6 @@ describe "LuckyRecord::Form" do
       form.nickname.is_a?(LuckyRecord::Field).should be_true
       form.name.value.should eq "someone"
       form.name.is_a?(LuckyRecord::FillableField).should be_true
-      form.name.is_a?(LuckyRecord::Fillable).should be_true
     end
   end
 

spec/virtual_spec.cr

@@ -16,7 +16,6 @@ end
 describe "virtual in forms" do
   it "is a FillableField" do
     form.password_confirmation.should be_a(LuckyRecord::FillableField(String?))
-    form.password_confirmation.should be_a(LuckyRecord::Fillable)
     form.password_confirmation.name.should eq(:password_confirmation)
     form.password_confirmation.form_name.should eq("virtual")
   end

src/lucky_record/fillable_field.cr

@@ -1,14 +1,4 @@
 require "./field"
 
-module LuckyRecord::Fillable
-  abstract def name
-  abstract def value
-  abstract def form_name
-  abstract def errors
-  abstract def valid?
-  abstract def changed?
-end
-
 class LuckyRecord::FillableField(T) < LuckyRecord::Field(T)
-  include LuckyRecord::Fillable
 end