Do not use charset loaded by fontconfig

Do not use charset loaded by fontconfig

The only use for storing charset loaded by fontconfig in font engines
is for determine if a font supports certain codepoint, however FreeType
already does that. The charset loaded is sometimes not complete, for
instance in fontconfig 2.9.0 it removed Apple Roman platform support
for cmap loading, thus results a regression in common symbol fonts
(Wingdings, Webdings) rendering. Because those symbol fonts produced
by Monotype only contain two cmap tables: Apple Roman and Microsoft
Symbol, since the Microsoft Symbol table has a weird 0xF000 offset,
we always fallback to the Apple Roman cmap table.

Removing freetype charset cache also make each font engine consuming
less memory.

Change-Id: I88f3f44981f3a1c517b84809a3f5b834ffff7681
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>