Remove Log4j v1 back-end

[vorburger]

As per https://github.com/apache/logging-log4j2/pull/629/files,

given that Flogger is a "library" (itself for logging, but whatever) and not an "application", given that Log4j v1 has been officially unmaintained for many years, and is partially broken on Java 9+, and has at least one known security vulnernability which will never be patched,

would you possibly consider simply entirely removing https://github.com/google/flogger/tree/master/log4j from this project,

given that there is https://github.com/google/flogger/tree/master/log4j2?

This is similar to e.g. netty/netty#11925 suggestion to an (otherwise unrelated) project.

[vorburger]

FTR: Google now no longer builds the Log4j (v1 not v2) backend internally, but the related code is currently still kept in this Git repository.