docs: Remove note about stringified options

I'm not entirely sure about this, I'm aware of the origin #3987 yet this PR I made proofs the opposite #12060 
Only thing I can think of right now that browser APIs get stringified, couldn't verify this so far, this #3834 is suggesting they get stringified and it's the reason the note was added to the docs in the first place..

Couldn't just leave my confusion alone and thought doing it in a PR might be useful if it's actually not valid anymore so it can be merged directly, otherwise feel free to reject it of course 👍

docs/docs/plugins.md

@@ -78,8 +78,6 @@ module.exports = {
 }
 ```
 
-Note that plugin options will be stringified by Gatsby, so they cannot be functions.
-
 ## Loading plugins from your local plugins folder
 
 Gatsby can also load plugins from the your local website plugins folder which is a folder named `plugins` in the website's root directory.