net-libs/webkit-gtk: Remove security vulnerable SLOT=2 and SLOT=3

net-libs/webkit-gtk: Remove security vulnerable SLOT=2 and SLOT=3

These slots are years old, unmaintained and vulnerable to hundreds of
bugs with a CVE assigned. Additionally there are some known unfixed
issues building against newer ICU. No interest in continued maintenance
of known hugely security vulnerable versions, thus with no consumers
remaining in main tree, this is going away now. Sorry.
If you need this, please think seriously ten times before you make it
available to yourself via an overlay, or use any software that requires
this security riddled version still.

Closes: https://bugs.gentoo.org/577068