GRAILS-11029 removeFrom missing check otherSide attribute

Made improvement on condition of removeFrom due to following exception during manyToMany relation removeFrom method invocation.

```
Cannot get property 'name' on null object. Stacktrace follows:
Message: Cannot get property 'name' on null object
```
see details 
http://stackoverflow.com/questions/21311359/domain-multiple-inheritance-and-manytomany-relation-removefrom-not-working