Remove --threshold argument

Tesseract is now included better thresholding (binarization) in v5. Users that have
thresholding issues should try that first. If we find further problems
this can be brought back as a plugin.

src/ocrmypdf/cli.py

@@ -287,14 +287,6 @@ Online documentation is located at:
         help="EXPERIMENTAL. Mask out any vector objects in the PDF so that they "
         "will not be included in OCR. This can eliminate false characters.",
     )
-    preprocessing.add_argument(
-        '--threshold',
-        action='store_true',
-        help=(
-            "EXPERIMENTAL. Threshold image to 1bpp before sending it to Tesseract "
-            "for OCR. Can improve OCR quality compared to Tesseract's thresholder."
-        ),
-    )
 
     ocrsettings = parser.add_argument_group("OCR options", "Control how OCR is applied")
     ocrsettings.add_argument(