configs/freescale_mpc8315erdb: remove defconfig

configs/freescale_mpc8315erdb: remove defconfig

This defconfig currently fails to build the Linux kernel:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/55306826

In addition, the U-Boot build had already been removed in commit
12c01e4a05c8e4f4e0c4e8b74fac7824efdc8b41
("configs/freescale_mpc8315erdb: remove U-Boot build"), back in
October 2016, and nobody bothered fixing it.

This defconfig was originally contributed and maintained by Gustavo
Zacarias, but he is no longer active in Buildroot, and nobody
expressed interest in this defconfig, so let's get rid of it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>