Remove pipeline variable unique validation

This causes unnecessary SQL queries, and is already checked in
`Gitlab::Ci::Pipeline::Chain::Build::Associations`.

Changelog: performance

app/models/ci/pipeline_variable.rb

@@ -8,7 +8,7 @@ module Ci
 
     alias_attribute :secret_value, :value
 
-    validates :key, uniqueness: { scope: :pipeline_id }
+    validates :key, presence: true
 
     def hook_attrs
       { key: key, value: value }

spec/models/ci/pipeline_variable_spec.rb

@@ -7,7 +7,7 @@ RSpec.describe Ci::PipelineVariable do
 
   it_behaves_like "CI variable"
 
-  it { is_expected.to validate_uniqueness_of(:key).scoped_to(:pipeline_id) }
+  it { is_expected.to validate_presence_of(:key) }
 
   describe '#hook_attrs' do
     let(:variable) { create(:ci_pipeline_variable, key: 'foo', value: 'bar') }