extensionPrefs: Remove version check from extensionAvailable()

extensionPrefs: Remove version check from extensionAvailable()

It is confusing to treat outdated extensions as if they didn't exist
in some places, but like any other extensions elsewhere. In particular
gnome-tweak-tool still allows to launch prefs for them while making
it clear to users that the extension will not work - opening the
list of installed extensions in that case is unexpected and confusing.
Just remove the version check for now, we will soon follow tweak-tool
and use it to disable the extension switch instead.

https://bugzilla.gnome.org/show_bug.cgi?id=736185