Remove os/slave.h and gems/report_rom_slave.h

[nfeske]

Those parts of the API were created out of intellectual curiosity but ultimately turned out to be dead ends. Hosting services as children has not become a natural pattern.

In the meantime, all former users of these mechanisms have vanished (#3512). Granted, there is qt_avplay in genode-world, but this application is actually merely reusing the Slave::Policy part of the API and can thereby easily be adapted to the use of the plain Child::Policy.

[nfeske]

This issue needs the adaptation of the following components:

• world/src/app/qt_avplay
• os/src/test/resource_yield
• os/src/test/dynamic_config

[nfeske]

The commits referenced above contain the remaining steps to solve this issue for the Genode repository.

Regarding qt_avplay in world, I have merely added a local copy of the original os/slave.h to the application for now. Down the road, we should better adjust the program similar to what I did in fad0ef5.