Revert #134 changes to message.py

[sloretz]

This reverts the changes to message.py from #134. This PR makes the codec always be registered so that messages generated with genpy 0.6.14 continue to work when used with Python 2 and newer genpy versions.

Closes #138

[sloretz]

@silentpony @apojomovsky @jaejyn @YiHerngOng @eggerk if any of you have an environment where the deserialization error still occurs, would you mind trying this change and reporting if it fixes it for you?

[eggerk]

Thank you for the quick fix!

I can confirm that this resolves the issue in my case.

[rovbo-maksim]

It fixes the problem for me, too.

[sloretz]

Thanks for the feedback! I tested locally by building custom messages with genpy 0.6.14 and running with 0.6.15 and this version. With 0.6.15 rostopic pub/rostopic echo the subscriber side silently fails and never prints any messages, while with this version it succeeds as expected. I'll merge an make a release into ROS Melodic.

[kvnptl]

Package version 0.6.16 solves the issue (for Error message: LookupError: unknown error handler name 'rosmsg') for python2. Thanks for the fix.