Remove API handler for Toolhive format

[dmartinol]

Still related to #76

• Removing format option for API data sources (either "" or "upstream")

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.14%. Comparing base (9744f94) to head (536280a).

Files with missing lines	Patch %	Lines
internal/sources/api.go	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
- Coverage   65.93%   65.14%   -0.80%     
==========================================
  Files          51       50       -1     
  Lines        2842     2677     -165     
==========================================
- Hits         1874     1744     -130     
+ Misses        833      807      -26     
+ Partials      135      126       -9     

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