[verdi branch]: CodeBuilder needs a way to remove keys

[DropD]

Summary

With the addition of the check for extraneous keys it becomes necessary to be able to remove them after new() complains.

Problem Illustration:

ipython >> builder = CodeBuilder()
ipython >> builder.label = ...
# ... lines and lines of generating and adding the other keys
ipython >> builder.extraneous_key = ...
ipython >> my_code = builder.new()
CodeValidationError ...
ipython >> # Now the builder object that has been prepared in arduous work has become useless
ipython >> # And we need to start from scratch!