Drop uniqueness validation of project_key

It's not necessary to enforce this validation because also project
full path is part of the email key.

ee/app/models/service_desk_setting.rb

@@ -8,7 +8,6 @@ class ServiceDeskSetting < ApplicationRecord
   validate :valid_issue_template
   validates :outgoing_name, length: { maximum: 255 }, allow_blank: true
   validates :project_key, length: { maximum: 255 }, allow_blank: true, format: { with: /\A[a-z0-9_]+\z/ }
-  validates_uniqueness_of :project_key, allow_nil: true
 
   def issue_template_content
     strong_memoize(:issue_template_content) do

ee/spec/models/service_desk_setting_spec.rb

@@ -9,7 +9,6 @@ describe ServiceDeskSetting do
     it { is_expected.to validate_presence_of(:project_id) }
     it { is_expected.to validate_length_of(:outgoing_name).is_at_most(255) }
     it { is_expected.to validate_length_of(:project_key).is_at_most(255) }
-    it { is_expected.to validate_uniqueness_of(:project_key).allow_nil }
     it { is_expected.to allow_value('abc123_').for(:project_key) }
     it { is_expected.not_to allow_value('abc 12').for(:project_key) }
     it { is_expected.not_to allow_value('Big val').for(:project_key) }