fontconfig: remove obsolete relative_symlinks class

fontconfig: remove obsolete relative_symlinks class

The fontconfig recipe inherited this because the symlink farm that it
creates in /etc/fonts/conf.d/ to /usr/share/fontconfig/conf.avail/ were
all absolute links, and this causes "problems" for the native sstate
code[1].

However, as of fontconfig 2.15[2] these links are created relative, so
this inherit is redundant.

[1] oe-core e478550c8cd ("openssl/fontconfig/bzip2: Use relative symlinks instead of absolute ones (using a new class)")
[2] fontconfig 5d954398 ("Create a symlink with relative path")

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>