Revert "cmake: introduction of ZephyrExtension package"

This reverts commit ac898bc49ea8387dc96e8ba7fc21d6d0ed12e34c.

This commit was merged by mistake, but in a way that was against the
decision of the TSC on May 20, 2020, which was to wait a week to let
people digest befor merging.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>