chore: Remove unused constants

[ohbarye]

Summary

I just found that those constants are not used anywhere, then just removed them.

• SELF_UPDATE_TARBALL_URL : it may be remain when removing self-update
• SELF_UPDATE_DOWNLOAD_FOLDER : it may be remain when removing self-update
• ACCESS_FILENAME : this seems to have never been used

Please feel free to close this if you won't need this kind of nitpicking pull request. :P

Test plan

Let me put search results.

~/.g/g/o/yarn ❯❯❯ ag (SELF_UPDATE_TARBALL_URL|SELF_UPDATE_DOWNLOAD_FOLDER|ACCESS_FILENAME) .                                                        ⏎ remove-unused-constants ✱
src/constants.js
26:export const SELF_UPDATE_TARBALL_URL = 'https://yarnpkg.com/latest.tar.gz';
27:export const SELF_UPDATE_DOWNLOAD_FOLDER = 'updates';
89:export const ACCESS_FILENAME = '.yarn-access';
~/.g/g/o/yarn ❯❯❯

[BYK]

Thanks a lot!

[ohbarye]

Thanks for your review!