Remove hard dependency on version 1.2.10 for log4net

The version of log4net is fixed to 1.2.10. Hence one is unable to
use the log4net extension in a project using a higher version of
log4net. I have removed this restriction.
Note that there is no reference to log4net here https://github.com/nhibernate/nhibernate-core/blob/master/src/NHibernate/NHibernate.csproj

and how they use it instead: https://github.com/nhibernate/nhibernate-core/blob/master/src/NHibernate/Logging.cs#L78-L100