x86/kvm: Remove defunct KVM_DEBUG_FS Kconfig

Remove KVM_DEBUG_FS, which can easily be misconstrued as controlling
KVM-as-a-host.  The sole user of CONFIG_KVM_DEBUG_FS was removed by
commit cfd8983f ("x86, locking/spinlocks: Remove ticket (spin)lock
implementation").

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200528031121.28904-1-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>