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

misc/completion/ocrmypdf.fish

@@ -29,7 +29,6 @@ complete -c ocrmypdf -s d -l deskew -d "fix small horizontal alignment skew"
 complete -c ocrmypdf -s c -l clean -d "clean document images before OCR"
 complete -c ocrmypdf -s i -l clean-final -d "clean document images and keep result"
 complete -c ocrmypdf -l remove-vectors -d "don't send vector objects to OCR"
-complete -c ocrmypdf -l threshold -d "threshold images before OCR"
 
 complete -c ocrmypdf -s f -l force-ocr -d "OCR documents that already have printable text"
 complete -c ocrmypdf -s s -l skip-ocr -d "skip OCR on pages that text, otherwise try OCR"