apis: drop check for volumes with user namespaces

The second phase of user namespaces support was related to supporting
only stateless pods.  Since the changes were accepted for the KEP, now
the scope is extended to support stateful pods as well.  Remove the
check that blocks creating PODs with volumes when using user namespaces.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

pkg/apis/core/validation/validation_test.go

@@ -21780,8 +21780,8 @@ func TestValidateHostUsers(t *testing.T) {
 			}},
 		},
 	}, {
-		name:    "hostUsers=false - unsupported volume",
-		success: false,
+		name:    "hostUsers=false - stateful volume",
+		success: true,
 		spec: &core.PodSpec{
 			SecurityContext: &core.PodSecurityContext{
 				HostUsers: &falseVar,
@@ -21794,7 +21794,6 @@ func TestValidateHostUsers(t *testing.T) {
 			}},
 		},
 	}, {
-		// It should ignore unsupported volumes with hostUsers=true.
 		name:    "hostUsers=true - unsupported volume",
 		success: true,
 		spec: &core.PodSpec{