configs/pandaboard_defconfig: drop defconfig

configs/pandaboard_defconfig: drop defconfig

The defconfig has failed to build since May with the change to GCC 13.x,
E.G.:

https://gitlab.com/buildroot.org/buildroot/-/jobs/6865597768

I no longer have access to a pandaboard.  Pandaboard/omap4 support has
recently been dropped from u-boot:
https://lists.denx.de/pipermail/u-boot/2024-July/558846.html

And (part of) the kernel support is also getting removed:
https://patchwork.kernel.org/project/linux-media/patch/815a789d-85a5-44a1-8b9c-429ac0101e3f@xs4all.nl/

So drop the defconfig here as well.

Signed-off-by: Peter Korsgaard <[email protected]>
Signed-off-by: Yann E. MORIN <[email protected]>