msys: Drop support for building in an MSYS environment

MSYS support was added as a stop-gap while native windows support was
unsupported. Now that Native windows support is stable we can drop
support for MSYS.

Dropping support for MSYS fixes #11260 and allows us to spend more
resources on native windows support.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>