Delete line-drawing fixtures and remove ligatures

The C/A/T didn't support line-drawing, and \D'...' sequences are treated
literally by ancient versions of troff(1), sans the leading backslash.

Ligatures are problematic because grops(1) uses absolute motions to draw
them, whereas dedicated glyphs were used by the C/A/T. There's no way of
fixing this, short of patching grops(1) to recognise "C ffi" as a single
character. Using multiple character sequences to "fake" ligatures proved
impossible because C/A/T commands are emitted boustrophedonically (where
movement/drawing sequences are reversed for every second line).

Rather than using crude hacks to "fix" this, we'll document it as a bug.
Luckily, Roff.js's canvas renderer isn't affected by this, as "C foobar"
is treated as "t foobar" without position changing (assuming "foobar" is
not a recognised character).