added delete fine tune model endpoint

[BrendanMartin]

Describe the change
With the deprecation of the fine-tunes endpoint, OpenAI moved the deletion of fine-tuned models to DELETE /models/{model}. This PR adds this new endpoint in models.go.

Describe your solution
I reworked the code present in fine_tunes.go that was already written for deleting fine-tuned models. Because OpenAI organized the new delete under the "models" prefix, this code is now located in the models.go file.

Tests
The tests used for the previous fine-tune model delete were reworked to use the new endpoint.

[codecov]

Codecov Report

Merging #497 (ac003ce) into master (3589837) will increase coverage by 0.15%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #497      +/-   ##
==========================================
+ Coverage   97.30%   97.46%   +0.15%     
==========================================
  Files          18       18              
  Lines         780      827      +47     
==========================================
+ Hits          759      806      +47     
  Misses         15       15              
  Partials        6        6              

Files Changed	Coverage Δ
embeddings.go	100.00% <100.00%> (ø)
models.go	100.00% <100.00%> (ø)

[sashabaranov]

Thank you so much!