chore: avoid double future

[fengjiachun]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

As the title said, the tables API does nod need to be an async method since it return a BoxStream

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.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 85.30%. Comparing base (0e05f85) to head (729552e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3890      +/-   ##
==========================================
- Coverage   85.65%   85.30%   -0.36%     
==========================================
  Files         952      952              
  Lines      163607   163558      -49     
==========================================
- Hits       140145   139522     -623     
- Misses      23462    24036     +574