x86/lib: Drop the unused return value from wbinvd_on_all_cpus()

Drop wbinvd_on_all_cpus()'s return value; both the "real" version and the
stub always return '0', and none of the callers check the return.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250522233733.3176144-3-seanjc@google.com