u-boot on qemu: remove currently unused x86 target

it doesn't build, at present, but isn't used by any
coreboot targets, so the build issue does not come up
during release builds, but i did find it laying around
during my audits.

x86 qemu is on todo for libreboot, on all x86 boards,
but the current config is broken, so: remove it.

it's very much a requirement that anything in lbmk should
work.

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/u-boot/qemu_x86_12mb/target.cfg

@@ -1,3 +0,0 @@
-tree="default"
-xtree="default"
-xarch="i386-elf"