refactor: remove "volar.usePrettier" command and docs adjustment

[yaegassy]

Description

The default value of prettier.disableLanguages has been changed in coc-prettier since "v9.2.0". Previously, vue was set to disable by default.

• REF:
  • Use latest code from prettier-vscode neoclide/coc-prettier#145
  • https://github.com/neoclide/coc-prettier/blob/master/package.json#L42

Because of this, if coc-prettier is installed, the built-in formatter of the Language Server (e.g. volar) will not be used, and prettier will be used preferentially.

For this reason, the volar.usePrettier command is unnecessary and has been removed.

If you want to use volar's built-in formatter

Added explanation to README.

[yaegassy]

The example of setting prettier.formatterPriority to -1 doesn't look so good...

For example, in the case of html files, prettier is not used and the coc-html (html-language-features) formatter is used. I think it would be better to disable prettier only for vue in the vue project.

It is better to change the README description to only prettier.disableLanguages description.