refactor: remove Vale workaround for StylesPath

[alexeagle]

Thanks @jdkato for the fix in errata-ai/vale@2139c41 so we no longer need to tell users about copying their configuration file to be relative to the styles folder.

Note, to be less breaking I didn't remove the older Vale versions. Users who pinned to those and set their StylesPath should still work, as this newly added environment variable won't be referenced.

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Type of change

• Refactor (a code change that neither fixes a bug or adds a new feature)

For changes visible to end-users

• Relevant documentation has been updated

Test plan

• Covered by existing test cases