chore: stop exposing num_partitions

[niebayes]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Since we only utilize the partition 0 (the first partition) of each Kafka topic, there's no need to expose the num_partitions to users through config file. If it's exposed, some users may set it to be greater than 1 and expect that there're multiple partitions per topic which we cannot provide.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (490312b) 85.55% compared to head (7c8f48d) 85.04%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3132      +/-   ##
==========================================
- Coverage   85.55%   85.04%   -0.51%     
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  Files         818      819       +1     
  Lines      133858   134018     +160     
==========================================
- Hits       114521   113976     -545     
- Misses      19337    20042     +705