Remove log code from library

[fwalch]

When using rust-zmq in an application, I noticed some log entries when dropping sockets. IMO this doesn't provide any useful information, so I removed it here. If you don't agree with this, just close the PR :)

Logging isn't used anywhere else in the library code, so this change also allows removing the dependency on the log crate from the non-dev section.

[rotty]

Thanks, that code has indeed long surpassed its usefulness!