Remove clean target from Makefile

[saulecabrera]

This commit removes the clean target from the Makefile:

• We no longer have to deal with downloading wasi-sdk
• Given (1), we can simply call cargo clean if we want to clean cargo artifacts.

Checklist

• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-core do not require updating CHANGELOG files.
• I've updated the relevant crate versions if necessary. Versioning policy for library crates
• I've updated documentation including crate documentation if necessary.