Revert "[UII] Fix client-side validation for agent policy timeout fie…

…lds (elastic#191674)" (elastic#194338)

## Summary

Resolves elastic#194221.

This PR reverts commit f29bf1c. Timeout
fields should be clearable with undefined value or zero.

x-pack/plugins/fleet/public/applications/fleet/sections/agent_policy/components/agent_policy_validation.tsx

@@ -38,20 +38,20 @@ export const agentPolicyFormValidation = (
     errors.namespace = [namespaceValidation.error];
   }
 
-  if (agentPolicy.unenroll_timeout !== undefined && agentPolicy.unenroll_timeout <= 0) {
+  if (agentPolicy.unenroll_timeout && agentPolicy.unenroll_timeout < 0) {
     errors.unenroll_timeout = [
       <FormattedMessage
         id="xpack.fleet.agentPolicyForm.unenrollTimeoutMinValueErrorMessage"
-        defaultMessage="Unenroll timeout must be an integer greater than zero"
+        defaultMessage="Unenroll timeout must be greater than zero."
       />,
     ];
   }
 
-  if (agentPolicy.inactivity_timeout !== undefined && agentPolicy.inactivity_timeout <= 0) {
+  if (agentPolicy.inactivity_timeout && agentPolicy.inactivity_timeout < 0) {
     errors.inactivity_timeout = [
       <FormattedMessage
         id="xpack.fleet.agentPolicyForm.inactivityTimeoutMinValueErrorMessage"
-        defaultMessage="Inactivity timeout must be an integer greater than zero"
+        defaultMessage="Inactivity timeout must be greater than zero."
       />,
     ];
   }