Remove warning for inactive request counter in active_requests method

[aniketmaurya]

What does this PR do?

Tracking the number of active requests is an opt-in feature and users should not see the warning if it's not enabled.

This PR removes a warning message from the active_requests method when the active request counter is not enabled.

/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/litserve/server.py:388: UserWarning: Active request counter is not enabled while using `on_request` callback hook. Please set track_requests=True in the LitServer.`

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89%. Comparing base (c539758) to head (2dd77be).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #466   +/-   ##
===================================
- Coverage    89%    89%   -0%     
===================================
  Files        37     37           
  Lines      2159   2158    -1     
===================================
- Hits       1914   1913    -1     
  Misses      245    245           

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