maxsize and maxver should be excluded from the loggers entry in config-get's output if destination is not a file

maxsize and maxver are always included in config-get's output and this is a regression (or a feature) gained in Kea 2.1.2. They should be excluded when the output is not a file since they don't make sense in that case.

The behavior is similar for other entries. For example, "lease-database" excludes user, password, port when type is memfile.