Remove redundant configs/birefnet.yaml; drop a stray --config example

BiRefNet is the default backend and --seg-backend switches segmenters, so the
birefnet.yaml config (a copy of default.yaml with backend: birefnet) no longer had
a purpose. configs/ now holds just default.yaml, the editable parameter template.
Also dropped the now-redundant --config from the chroma-debug README example.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -103,7 +103,6 @@ D:\MiniConda\envs\lightML\python.exe -m bgfilter.cli `
   --input-dir Samples `
   --output-dir Outputs\chroma `
   --debug-dir Outputs\chroma_debug `
-  --config configs\default.yaml `
   --matting-method chroma `
   --device cpu
 ```