drop *.otb for X11: it's redundant with *.bdf

> X11 (Unix/Linux): The X consortium have
> devised their own format which they call
> "OpenType Bitmap" with extension `.otb`.

https://fontforge.github.io/bitmaponlysfnt.html

README.md

@@ -98,7 +98,6 @@ Other platforms:
 
 * Try using the font files in these subfolders:
   * `dfont/` - Apple bitmap only sfnt (dfont)
-  * `otb/`   - X11 bitmap only sfnt (otb)
   * `fnt/`   - Win FNT
   * `pdb/`   - Palm OS Bitmap
   * `pt3/`   - PS Type3 Bitmap

Rakefile

@@ -218,7 +218,6 @@ CLOBBER.include '.portable', 'pcf'
 
 FONTFORGE_FORMATS = [
   'dfont',    # Apple bitmap only sfnt (dfont)
-  'otb',      # X11 bitmap only sfnt (otb)
   'fnt',      # Win FNT
   'pdb',      # Palm OS Bitmap
   'pt3',      # PS Type3 Bitmap