Remove APT from sysupdate

[jumpyvi]

Lets remove apt/apt-get from sysupdate, and move it the elementary-sdk sysexts.

We can add /usr/bin/apt and /usr/bin/apt-get that would print something like This system uses atomic updates, check the docs to learn how to install software and to apply system updates at https://elementarydoc.whereeverthedocsare

[ryonakano]

@jumpyvi The io.elementary.apt-shim package is now available on Daily PPA. Would you check if everything works as expected when adding that package to the image (only in sysupdate one, should not be in the classic one)?

[jumpyvi]

Yeah, I should check sometime this week