Use Environment#deployment_platform method instead

As this method does have `deployment_platform(environment: self.name)`,
which is what the form EE needs. Allows us to remove two overrides in
EE.

app/models/environment.rb

@@ -155,11 +155,11 @@ class Environment < ApplicationRecord
   end
   def has_terminals?
-    project.deployment_platform.present? && available? && last_deployment.present?
+    deployment_platform.present? && available? && last_deployment.present?
   end
   def terminals
-    project.deployment_platform.terminals(self) if has_terminals?
+    deployment_platform.terminals(self) if has_terminals?
   end
   def has_metrics?

spec/models/environment_spec.rb

@@ -592,9 +592,7 @@ describe Environment do
       shared_examples 'same behavior between KubernetesService and Platform::Kubernetes' do
         it 'returns the terminals from the deployment service' do
-          deployment_platform_target = Gitlab.ee? ? environment : project
-
-          expect(deployment_platform_target.deployment_platform)
+          expect(environment.deployment_platform)
             .to receive(:terminals).with(environment)
             .and_return(:fake_terminals)