ci: remove PRs refspec from the ITs docker images job

[kuisathaverat]

What does this PR do?

Remove the refspec to map the PR.

Why is it important?

This refspec is causing issues with one PR, remove the refspec should resolve the issue.

18:10:21   > git fetch --tags --progress --depth=1 [email protected]:elastic/beats.git +refs/heads/*:refs/remotes/origin/* +refs/pull/*/head:refs/remotes/origin/pr/* # timeout=10
18:10:22  ERROR: Error fetching remote repo 'origin'
18:10:22  hudson.plugins.git.GitException: Failed to fetch from [email protected]:elastic/beats.git
18:10:22  	at hudson.plugins.git.GitSCM.fetchFrom(GitSCM.java:909)
18:10:22  	at hudson.plugins.git.GitSCM.retrieveChanges(GitSCM.java:1131)
18:10:22  	at hudson.plugins.git.GitSCM.checkout(GitSCM.java:1167)
18:10:22  	at org.jenkinsci.plugins.workflow.steps.scm.SCMStep.checkout(SCMStep.java:125)
18:10:22  	at org.jenkinsci.plugins.workflow.cps.CpsScmFlowDefinition.create(CpsScmFlowDefinition.java:155)
18:10:22  	at org.jenkinsci.plugins.workflow.cps.CpsScmFlowDefinition.create(CpsScmFlowDefinition.java:69)
18:10:22  	at org.jenkinsci.plugins.workflow.job.WorkflowRun.run(WorkflowRun.java:309)
18:10:22  	at hudson.model.ResourceController.execute(ResourceController.java:97)
18:10:22  	at hudson.model.Executor.run(Executor.java:428)
18:10:22  Caused by: hudson.plugins.git.GitException: Command "git fetch --tags --progress --depth=1 [email protected]:elastic/beats.git +refs/heads/*:refs/remotes/origin/* +refs/pull/*/head:refs/remotes/origin/pr/*" returned status code 128:
18:10:22  stdout: 
18:10:22  stderr: fatal: Cannot fetch both refs/heads/pr/19387 and refs/pull/19387/head to refs/remotes/origin/pr/19387
18:10:22  
18:10:22  	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandIn(CliGitAPIImpl.java:2430)
18:10:22  	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandWithCredentials(CliGitAPIImpl.java:2044)
18:10:22  	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.access$500(CliGitAPIImpl.java:81)
18:10:22  	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl$1.execute(CliGitAPIImpl.java:569)
18:10:22  	at hudson.plugins.git.GitSCM.fetchFrom(GitSCM.java:907)

https://stackoverflow.com/questions/21998181/git-fetch-failed-refs-remotes-origin-pr-34-tracks-both/22000105#22000105

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

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• Build Cause: [Pull request #19681 opened]

• Start Time: 2020-07-06T16:47:35.828+0000

• Duration: 29 min 29 sec