Use FluentLogger instead of slf4j

The Gerrit-project moved to FluentLogger last year [1]. The
websession-plugin was still using slf4j.

This change exchanges the slf4j logger with FluentLogger.

[1] https://gerrit-review.googlesource.com/c/gerrit/+/176930

Change-Id: Ifa0f78e3d1ee480b07f0f522d55bcf085991c3da
1 file changed