Fix symbol cmap handling

A while back we removed Apple Roman encoding support.  This broke
symbol fonts (Wingdings, etc) because those fonts come with two
cmaps:

  1) platform=1,encoding=0, aka Apple Roman, which maps identity,
  2) platform=3,encoding=0, aka MS Symbol font

Now, the reason the Apple Roman removal "broke" these fonts is
obvious, and for the better: these fonts were mapping ASCII and
other Latin chars to symbols.

The reason the fonts didn't work anymore, however, is that we were
mishandling the MS symbol-font cmaps.  In their modern incarnation
they are like regular non-symbol-font cmap that map PUA codepoints
to symbols.  We want to expose those as such.  Hence, this change
just removes the special-handling for that.

Now, the reason this confusion happened, if I was to guess, is either
that FreeType docs are wrong saying that FT_ENCODING_MS_SYMBOL is
the "Microsoft Symbol encoding, used to encode mathematical symbols":

  http://www.kostis.net/charsets/symbol.htm

or maybe it started that way, but turned into also mapping MS symbol-
font cmaps, which is a completely different thing.  At any rate, I
don't know if there are any fonts that use this thing these days, but
the code here didn't seem to produce charset for any font.  By now I'm
convinced that this change is the Right Thing to do.  The MS Symbol
thing was called AdobeSymbol in our code by the way.

This fixes the much-reported bug that windings, etc are not usable
with recent fontconfig:
https://bugs.freedesktop.org/show_bug.cgi?id=58641

Now I see PUA mappings reported for Wingdings.

This also fixes:
Bug 48947 - Drop the non-Unicode cmap support gradually
https://bugs.freedesktop.org/show_bug.cgi?id=48947
since the AdobeSymbol was the last non-Unicode cmap we were
trying to parse (very incorrectly).

Lots of code around this change can be simplified.  I'll push those
out (including removing the table itself) in subsequent changes.

src/fcfreetype.c

@@ -2206,7 +2206,7 @@ static const FcCharMap AdobeSymbol = {
 
 static const FcFontDecode fcFontDecoders[] = {
     { ft_encoding_unicode,	0,		(1 << 21) - 1 },
-    { ft_encoding_symbol,	&AdobeSymbol,	(1 << 16) - 1 },
+    { ft_encoding_symbol,	0,		(1 << 16) - 1 },
 };
 
 #define NUM_DECODE  (int) (sizeof (fcFontDecoders) / sizeof (fcFontDecoders[0]))