cli: remove --source command line option

This was deprecated in favor of always having the source in the
manifest. Remove the command line option and the corresponding
code that would override the sources definitions.
Update the docs accordingly.

docs/osbuild.1.rst

@@ -39,8 +39,6 @@ is not listed here, **osbuild** will deny startup and exit with an error.
 -h, --help                      print usage information and exit immediately
 --store=DIR                     directory where intermediary file system trees
                                 are stored
---sources=PATH                  json file containing a dictionary of source
-                                configuration
 --secrets=PATH                  json file containing a dictionary of secrets
                                 that are passed to sources
 -l DIR, --libdir=DIR            directory containing stages, assemblers, and