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.

misc/completion/ocrmypdf.bash

@@ -40,7 +40,7 @@ _ocrmypdf()
             --sidecar --version --jobs --quiet --verbose --title --author
             --subject --keywords --rotate-pages --remove-background --deskew
             --clean --clean-final --unpaper-args --oversample --remove-vectors
-            --threshold --force-ocr --skip-text --redo-ocr
+            --force-ocr --skip-text --redo-ocr
             --skip-big --jpeg-quality --png-quality --jbig2-lossy
             --max-image-mpixels --tesseract-config --tesseract-pagesegmode
             --help --tesseract-oem --pdf-renderer --tesseract-timeout