KVM: x86: Drop the now unused KVM_X86_DISABLE_VALID_EXITS

Drop the KVM_X86_DISABLE_VALID_EXITS definition, as it is misleading, and
unused in KVM *because* it is misleading.  The set of exits that can be
disabled is dynamic, i.e. userspace (and KVM) must check KVM's actual
capabilities.

Suggested-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20241128013424.4096668-16-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>