Remove unused EE::GitPushService

What does this MR do?

Removes the unused EE::GitPushService, the code for this lives in ee/app/services/ee/git/branch_push_service.rb since https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10408.

Ref: https://gitlab.com/gitlab-org/gitlab-ee/issues/12464

The API for Git::BranchPushService is still the same, but I manually verified I'm not breaking anything with rake db:seed_fu FILTER=cycle_analytics SEED_CYCLE_ANALYTICS=1 and the patch below:

diff --git i/db/fixtures/development/17_cycle_analytics.rb w/db/fixtures/development/17_cycle_analytics.rb
index 78ceb74da65..8ed46f81f1b 100644
--- i/db/fixtures/development/17_cycle_analytics.rb
+++ w/db/fixtures/development/17_cycle_analytics.rb
@@ -141,18 +141,19 @@ def mention_in_commits(issues)
 
       branch_name = filename = "#{FFaker::Product.brand}-#{FFaker::Product.brand}-#{rand(1000)}"
 
-      issue.project.repository.add_branch(@user, branch_name, 'master')
+      # issue.project.repository.add_branch(@user, branch_name, 'master')
 
-      commit_sha = issue.project.repository.create_file(@user, filename, "content", message: "Commit for #{issue.to_reference}", branch_name: branch_name)
-      issue.project.repository.commit(commit_sha)
+      # commit_sha = issue.project.repository.create_file(@user, filename, "content", message: "Commit for #{issue.to_reference}", branch_name: branch_name)
+      # issue.project.repository.commit(commit_sha)
 
       Git::BranchPushService.new(
         issue.project,
         @user,
         oldrev: issue.project.repository.commit("master").sha,
-        newrev: commit_sha,
+        newrev: issue.project.repository.commit("master").sha,
         ref: 'refs/heads/master'
       ).execute
+      exit
 
       branch_name
     end

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team