scripts: Remove checkconfig.py

This is an old script for finding references to undefined Kconfig
symbols that assumes Kconfiglib 1 (an older API).

There's now a different check for references to undefined symbols (see
commit 1d0834b35f ("checks: kconfig: Check for references to undefined
Kconfig symbols") in the ci-tools repo).

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>