Follows Firefox bug: CSS injected should be removable without page reload

[fregante]

This polyfill currently matches Firefox implementation, so it's compliant.

with browser.contentScripts.register in Firefox, the changes are always applied on the next reload, both on register and unregister
Originally posted in openstyles/stylus#287 (comment)

However Firefox might change this: https://bugzilla.mozilla.org/show_bug.cgi?id=1423323

When they do, this issue will become a bug, and can't be fixed.