lib,test: remove unneeded escaping of /

The `/` character does not need to be escaped when occurring inside a
character class in a regular expression. Remove such instances of
escaping in the code base.

PR-URL: https://github.com/nodejs/node/pull/9485


Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>