remove source command line line and SubiquityModel argument

nothing has used this value for some considerable time.

subiquity/cmd/server.py

@@ -37,9 +37,6 @@ def make_server_args_parser():
     parser.add_argument('--machine-config', metavar='CONFIG',
                         dest='machine_config',
                         help="Don't Probe. Use probe data file")
-    parser.add_argument('--source', default=[], action='append',
-                        dest='sources', metavar='URL',
-                        help='install from url instead of default.')
     parser.add_argument('--bootloader',
                         choices=['none', 'bios', 'prep', 'uefi'],
                         help='Override style of bootloader to use')

subiquity/models/subiquity.py

@@ -102,7 +102,7 @@ class SubiquityModel:
 
     target = '/target'
 
-    def __init__(self, root, sources=()):
+    def __init__(self, root):
         self.root = root
         if root != '/':
             self.target = root

subiquity/server/server.py

@@ -197,7 +197,7 @@ class SubiquityServer(Application):
         root = '/'
         if self.opts.dry_run:
             root = os.path.abspath('.subiquity')
-        return SubiquityModel(root, self.opts.sources)
+        return SubiquityModel(root)
 
     def __init__(self, opts, block_log_dir):
         super().__init__(opts)