Deprecate net-im/skype4

It is End of Life since 2017 [1]. Also deprecate linux-skype_oss_wrapper
which is only useful for skype4.

[1] https://answers.microsoft.com/en-us/skype/forum/skype_linux-skype_startms-skype_installms/end-of-life-of-skype-for-linux-43/70fe7036-ddf3-4fa3-9c06-1bca3ec0226c