[kubectl-plugin] Remove ephemeral storage check

[win5923]

Why are these changes needed?

I think the util.ValidateResourceQuantity already handles empty string values, so we can remove this.

kuberay/kubectl-plugin/pkg/util/validation.go

Lines 11 to 14 in b16fb3f

	func ValidateResourceQuantity(value string, name string) error {
	if value == "" {
	return nil
	}

Related issue number

Checks

• I've made sure the tests are passing.
• Testing Strategy
  • Unit tests
  • Manual tests
  • This PR is not tested :(