[SPARK-25016][INFRA][FOLLOW-UP] Remove leftover for dropping Hadoop 2.6 in Jenkins's test script

### What changes were proposed in this pull request?

This PR proposes to remove the leftover. After #22615, we don't have Hadoop 2.6 profile anymore in master.

### Why are the changes needed?

Using "test-hadoop2.6" against master branch in a PR wouldn't work.

### Does this PR introduce any user-facing change?

No (dev only).

### How was this patch tested?

Manually tested at #26707 and Jenkins build will test.

Without this fix, and hadoop2.6 in the pr title, it shows as below:

```
========================================================================
Building Spark
========================================================================
[error] Could not find hadoop2.6 in the list. Valid options  are dict_keys(['hadoop2.7', 'hadoop3.2'])
Attempting to post to Github...
```

Closes #26708 from HyukjinKwon/SPARK-25016.

Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

Author: HyukjinKwon

dev/run-tests-jenkins.py

@@ -176,8 +176,6 @@ def main():
     if "test-maven" in ghprb_pull_title:
         os.environ["AMPLAB_JENKINS_BUILD_TOOL"] = "maven"
     # Switch the Hadoop profile based on the PR title:
-    if "test-hadoop2.6" in ghprb_pull_title:
-        os.environ["AMPLAB_JENKINS_BUILD_PROFILE"] = "hadoop2.6"
     if "test-hadoop2.7" in ghprb_pull_title:
         os.environ["AMPLAB_JENKINS_BUILD_PROFILE"] = "hadoop2.7"
     if "test-hadoop3.2" in ghprb_pull_title: