Remove $FZF_PREVIEW_PIXEL_{WIDTH,HEIGHT} (#2544)

They are not neccessary because we can use a program such as chafa that
can resize images by the terminal columns and lines.

bin/fzf-preview.sh

@@ -2,6 +2,10 @@
 #
 # The purpose of this script is to demonstrate how to preview a file or an
 # image in the preview window of fzf.
+#
+# Dependencies:
+# - https://github.com/hpjansson/chafa
+# - https://github.com/sharkdp/bat
 
 file=$1
 type=$(file --mime-type "$file")
@@ -23,8 +27,8 @@ elif [[ $KITTY_WINDOW_ID ]]; then
   # you have to use 'stream'
   kitty icat --clear --transfer-mode=memory --stdin=no --place="${FZF_PREVIEW_COLUMNS}x${FZF_PREVIEW_LINES}@0x0" "$file" | sed \$d
   echo -en "\e[m"
-elif [[ -n $FZF_PREVIEW_PIXEL_WIDTH ]]; then
-  convert "$file" -resize "${FZF_PREVIEW_PIXEL_WIDTH}x${FZF_PREVIEW_PIXEL_HEIGHT}>" -dither FloydSteinberg sixel:-
+elif command -v chafa > /dev/null; then
+  chafa -f sixel -s "${FZF_PREVIEW_COLUMNS}x${FZF_PREVIEW_LINES}" "$file"
 else
   file "$file"
 fi