build: stop defining unused VERSION

build: stop defining unused VERSION

Scons and autotools don't define it, and as of last commit nothing
uses it.

`VERSION` is also a generic enough name that something somewhere will
eventually clash, and we don't want to repeat the LLVM `DEBUG` fiasco.

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>