Drop code that checks whether a connection pools jars are present and leave to ConnectionPoolFactory classes

[andyjefferson]

We have some code in ConnectionFactoryImpl, that is historical from the days before specifying ConnectionPoolFactory using the plugin mechanism, which checks for existence of DBCP, C3P0, BoneCP etc and falls back to whichever is present. We should just fall back to dbcp2-builtin when not specified and drop this code.