Check: Debug code has been left behind error_log

[davidperezgar]

We warn the developers to not use logs functions in final plugins.

As we say while reviewing, we warn them:

During development it is common to use functions such as error_log or var_dump to check the value or state of variables. 
Sometimes we leave this code by mistake in our final version, please take a look at it. 

I see that is already implemented in WPCS with DevelopmentFunctionsSniff. So, let's use it. It should have a 6 severity as it's not a blocker for wordpress.org submission.