[INTERNAL] MessageBox: remove wrong jsDoc @return of static methods
[INTERNAL] MessageBox: remove wrong jsDoc @return of static methods

Closes: https://github.com/SAP/openui5/pull/1943

See issue https://github.com/SAP/openui5/issues/1938
See jsbin http://jsbin.com/livelev/edit?html,console,output

Change-Id: Ic68f9e0a2877c25c09f30d8e558fcc4f01ba3ce8

SAP/openui5,cschuff/openui5,SAP/openui5,cschuff/openui5,SAP/openui5,SAP/openui5,cschuff/openui5