x86/bugs: Remove its=stuff dependency on retbleed

Allow ITS to enable stuffing independent of retbleed. The dependency is only
on retpoline. It is a valid case for retbleed to be mitigated by eIBRS while
ITS deploys stuffing at the same time.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250611-eibrs-fix-v4-6-5ff86cac6c61@linux.intel.com