rewriter: remove never-used default None when parsing arguments

The add_arguments function is always called with a formatter in
mesonmain.py, and if it were not, then it would be incorrect when
calling argparse itself -- because formatter_class cannot be None, and
defaults to its own builtin one. This violates None-safety.

mesonbuild/rewriter.py

@@ -25,7 +25,7 @@ class RewriterException(MesonException):
 
 # Note: when adding arguments, please also add them to the completion
 # scripts in $MESONSRC/data/shell-completions/
-def add_arguments(parser, formatter=None):
+def add_arguments(parser, formatter):
     parser.add_argument('-s', '--sourcedir', type=str, default='.', metavar='SRCDIR', help='Path to source directory.')
     parser.add_argument('-V', '--verbose', action='store_true', default=False, help='Enable verbose output')
     parser.add_argument('-S', '--skip-errors', dest='skip', action='store_true', default=False, help='Skip errors instead of aborting')