tools/sched_ext: Remove redundant SCX_ENQ_IMMED compat definition

tools/sched_ext: Remove redundant SCX_ENQ_IMMED compat definition

compat.bpf.h defined a fallback SCX_ENQ_IMMED macro using
__COMPAT_ENUM_OR_ZERO(). After 6bf36c68b0a2 ("tools/sched_ext:
Regenerate autogen enum headers") added SCX_ENQ_IMMED to the autogen
headers, including both triggers -Wmacro-redefined warnings.

The autogen definition through const volatile __weak already resolves to
0 on older kernels, providing the same backward compatibility. Remove
the now-redundant compat fallback.

Fixes: 6bf36c68b0a2 ("tools/sched_ext: Regenerate autogen enum headers")
Link: https://lore.kernel.org/r/20260326100313.338388-1-zhaomzhao@126.com
Reported-by: Zhao Mengmeng <zhaomengmeng@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>