TSS-41: Remove unused business_process attribute from Flex::Case (#79)

This PR removes the unused `business_process` attribute from the
`Flex::Case` class as it appears to be unused throughout the codebase.

## Changes
- Removed line `protected attr_accessor :business_process` from
`app/models/flex/case.rb`

---------

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: lorenyu

app/models/flex/case.rb

@@ -25,8 +25,6 @@ class Case < ApplicationRecord
 
     attribute :business_process_current_step, :string
 
-    protected attr_accessor :business_process
-
     # Closes the case, changing its status to 'closed'.
     #
     # @return [Boolean] True if the save was successful

spec/dummy/app/models/passport_case.rb

@@ -1,7 +1,3 @@
 class PassportCase < Flex::Case
   readonly attribute :passport_id, :string, default: -> { SecureRandom.uuid } # always defaults to a new UUID
-
-  def business_process
-    PassportBusinessProcess
-  end
 end

spec/dummy/app/models/test_case.rb

@@ -1,7 +1,4 @@
 class TestCase < Flex::Case
   # A simple test case to test the Case abstract class functionality
   # This class should not have any custom functionality added to it
-  def business_process
-    TestBusinessProcess
-  end
 end