arm: Remove kernel config APALIS-IMX6

It reference to a non-existant dts file apalis-imx6.dts so unlikekly to compile.
Aldo IMX6 support is in GENERIC so anyone interested in this board should
make it work with GENERIC kernel (if that's not already the case).

Sponsored by: Beckhoff Automation GmbH & Co. KG
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D39086