Remove portable prebuilts: disable building Crossgen2 pack

[dagood]

For #1937.

Building the Crossgen2 pack from source has a few issues:

Self-contained packages like this one bring down lots of prebuilts. We can disable
self-contained, but this only fixes this some prebuilts, not all. See
#1937. Also, disabling self-contained makes the
source-built Crossgen2 pack different from the Microsoft build's and potentially not work in the
expected scenarios.

Even if we managed to build it without prebuilts, there is no mechanism to distribute the
Crossgen2 pack in distro packages. This is the same problem as runtime packs. See
#1568

Running local build.