remove unnecessary if block from nvm get artifact compression

[imevanc]

Changes

• Remove an unnecessary if block from the get_artifact_compression function.

Reasons

• Given that COMPRESSION is defaulted to tar.gz, we don't need to add another condition that assigns tar to the COMPRESSION variable.
• I have experienced issues with the condition of the removed if block on M2 max macbooks.

[ljharb]

it defaults to tar dot G z, and these lines set it to tar dot X z.

It's an entirely different format, and is quite intentional so the smaller xz format can be downloaded instead.

[ljharb]

If you're experiencing issues, the best thing is to first file an issue rather than file an unsolicited PR removing lines that are there for a reason :-) happy to help if you file one!

[imevanc]

I will open an issue shortly, thank you!