Remove gracedb option from bilby_pipe

Once #79 (closed) is completed, there will be a way to generate an ini file based on the gracedb. As such, there is no need to have a --gracedb option - and indeed it will not work as expected since the prior will not be automatically inferred.

Instead, we will

• Deprecate the --gracedb flag
• Remove the X509 checking (which will no longer be used)
• Update the documentation to show how to work with gracedb

This will simplify things internally.