Remove old benchmarks

[marcotc]

What does this PR do?

This PR removes the Circle step called benchmark-2.3.

It indirectly reverts the PR that introduced them, #482, which means this PR also remove gems that were installed for this benchmark (ruby-prof).

Motivation:

This benchmark was introduced in #482, but we haven't inspected the results in quite some time, and have since moved to use our own internal benchmark platform.

Additional Notes:

sqlite3 was also introduced by #482, but it is no longer part of the default Gemfile.

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b4c9019) 98.23% compared to head (7d17fd9) 98.22%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3265      +/-   ##
==========================================
- Coverage   98.23%   98.22%   -0.01%     
==========================================
  Files        1253     1253              
  Lines       72393    72393              
  Branches     3393     3393              
==========================================
- Hits        71112    71111       -1     
- Misses       1281     1282       +1     

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