[FIX] disable benchmark code

[mengxr]

I forgot to disable the benchmark code in #2937, which increased the Jenkins build time by couple minutes.

@aarondav

[aarondav]

LGTM. This patch decreases build time by roughly 10 minutes. Good job!

[SparkQA]

Test build #22424 has started for PR 2990 at commit c58f070.

• This patch merges cleanly.

[SparkQA]

Test build #22424 has finished for PR 2990 at commit c58f070.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test FAILed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/22424/
Test FAILed.

[mengxr]

The failed test from streaming is a known flaky test. @tdas

I've merged this one into master (because it will speed up Jenkins builds).