node: Remove outdated caveat as of #22408

This PR removes an outdated caveat when building node `--with-npm`.

#### Rationale

When building the node formula `--with-npm` (which is the default), Homebrew still displays the following caveat:

> If you update npm itself do NOT use the `npm upgrade` command
> Instead execute:
> `npm install -g npm@latest`

The caveat seems confusing/misleading to me in the following ways:

* As @ijy [states](https://github.com/Homebrew/homebrew/issues/22408#issuecomment-56687851) in #22408, the workaround has become obsolete since PR #28075.<br />The latter fixes both #1564 and #22408; I figure this makes the caveat obsolete, too.

* Also, nitpicky me says there is no such thing as `npm upgrade`. (Did you mean `npm update`?)

#### Please review

I have left the [second caveat](https://github.com/claui/homebrew/blob/64a4f4ca8d0be04c912b5e632375b06669cc9b10/Library/Formula/node.rb#L97-L99) untouched. I assume it is still relevant?
Could anyone with more in-depth knowledge **please review** and confirm that it’s OK to leave it that way?

Thanks in advance!