configs/odroidc2: remove the defconfig

The odroidc2 defconfig is using gcc 4.9 version in order to build
uboot (2015.01) and kernel (3.14.79) for the board.

We are going to remove gcc 4.9 version in Buildroot and this
defconfig is the only remaining one using this version.

Since we don't have the board, we can't update the defconfig with
newer bootloader and kernel version.

A new defconfig for the odroidc2 board are welcome as soon as
it use a newer uboot and kernel version.

Remove the defconfig from the gitlab yaml.

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Dagg Stompler <daggs@gmx.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>