Remove unsupported providerImportSource option (#69609)
While working on applying custom components to nested MDX files using
`MDXProvider`, I noticed an outdated comment in the Next.js MDX README.
The comment references the `providerImportSource` option, which is no
longer supported. This option removed, as explained by @leerob
[here](https://github.com/vercel/next.js/issues/54212#issuecomment-1803973892).
This PR updates the Next.js MDX README by removing the outdated
reference to the `providerImportSource` option.
ccs:
@leerob
@wooorm
@remcohaszing
@manovotny
Co-authored-by: JJ Kasper <jj@jjsweb.site>
