fix: remove refspec because is no longer needed

[kuisathaverat]

What does this PR do?

• remove a refspec parameter not needed and that causes an issue (ci: remove PRs refspec from the ITs docker images job beats#19681).

Why is it important?

To grab these two refspecs in the same command can generate a conflict like we saw in elastic/beats#19681. The solution is to split the command in two, and we made it time ago,
we have the checkout that takes the regular refspec (+refs/heads/:refs/remotes/origin/) and fetchPullRefs(refspec) that tech the pull request references.

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #658 opened]

• Start Time: 2020-07-07T09:55:20.990+0000

• Duration: 11 min 14 sec

Test stats 🧪

Test	Results
Failed	0
Passed	676
Skipped	9
Total	685

[kuisathaverat]

This causes that a downstream job does not find the commit SHA, so we need the refspec

[2020-07-08T09:15:45.291Z] using credential f6c7695a-671e-4f4f-a331-acdce44ff9ba
[2020-07-08T09:15:45.329Z] Cloning the remote Git repositor
[2020-07-08T09:15:45.340Z] Cloning repository [email protected]:elastic/apm-integration-testing.git
[2020-07-08T09:15:45.361Z]  > git init /var/lib/jenkins/workspace/_integration-test-downstream_7.x/src/github.com/elastic/apm-integration-testing # timeout=10
[2020-07-08T09:15:45.371Z] [WARNING] Reference path does not exist: /var/lib/jenkins/apm-integration-testing.git
[2020-07-08T09:15:45.372Z] Fetching upstream changes from [email protected]:elastic/apm-integration-testing.git
[2020-07-08T09:15:45.372Z]  > git --version # timeout=10
[2020-07-08T09:15:45.385Z] using GIT_SSH to set credentials GitHub user @elasticmachine SSH key
[2020-07-08T09:15:45.389Z]  > git fetch --tags --progress -- [email protected]:elastic/apm-integration-testing.git +refs/heads/*:refs/remotes/origin/* # timeout=10
[2020-07-08T09:15:46.275Z]  > git config remote.origin.url [email protected]:elastic/apm-integration-testing.git # timeout=10
[2020-07-08T09:15:46.278Z]  > git config --add remote.origin.fetch +refs/heads/*:refs/remotes/origin/* # timeout=10
[2020-07-08T09:15:46.285Z]  > git config remote.origin.url [email protected]:elastic/apm-integration-testing.git # timeout=10
[2020-07-08T09:15:46.292Z] Fetching upstream changes from [email protected]:elastic/apm-integration-testing.git
[2020-07-08T09:15:46.292Z] using GIT_SSH to set credentials GitHub user @elasticmachine SSH key
[2020-07-08T09:15:46.295Z]  > git fetch --tags --progress -- [email protected]:elastic/apm-integration-testing.git +refs/heads/*:refs/remotes/origin/* # timeout=10
[2020-07-08T09:15:46.857Z]  > git rev-parse ea838c80672c8929e690692fa13e4696cf952a6c^{commit} # timeout=10
[2020-07-08T09:15:46.869Z]  > git rev-parse origin/ea838c80672c8929e690692fa13e4696cf952a6c^{commit} # timeout=10
[2020-07-08T09:15:46.885Z]  > git rev-parse ea838c80672c8929e690692fa13e4696cf952a6c^{commit} # timeout=10
Couldn't find any revision to build. Verify the repository and branch configuration for this job.