Docs: Remove outdated info on Skype highlighting

The snippet provided in the docs about how to prevent the Skype
plugin from formatting numbers on a web pages is outdated and no
longer works.

Nowadays, the Skype plugin makes it more difficult to overwrite
its injected styles (class names include timestamps, e.g.:
`skype_pnh_print_container_1381964168`, `!important` is used, etc.).

Providing a solution for this use case isn't viable. Also, users
that install such a plugin usually expect the behavior.

Close #1454.