Remove variant selection from Create a new Test Set with JSON/CSV

[kangsuhyun-yanolja]

Is your feature request related to a problem? Please describe.
The variant dropdown is not visible in the 'Create a New Test Set with JSON/CSV' section and may not be necessary there. The dropdown seems more appropriate for the Endpoint section, so I suggest removing it from the 'Create a New Test Set with JSON/CSV' section.

Describe the solution you'd like
Remove the variant selection from the 'Create a New Test Set with JSON/CSV' section.

Additional context
I encountered this issue while working on PR #605 and have already removed it in that PR.
Please let me know if the variant should not be removed.

[mmabrouk]

Thanks a lot @kangsuhyun-yanolja for noticing and solving this!