Remove unused ParameterAggregator class
Do not loose AllowPartiallyTrustedCallersAttribute after merge (NH-3340)

- NHibernate.dll is marked as SecurityRulesAttribute(SecurityRuleSet.Level1)
- Switch from ILMerge to ILRepack (https://github.com/gluck/il-repack) ver 1.20 to preserve AllowPartiallyTrustedCallersAttribute when merge assemblies