games/openra: de-duplicate dllmap in *.config files

SharpFont.dll.config already has os="freebsd" line but not os="dragonfly".
Due to symlinks this can lead to

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<dllmap dll="freetype6" os="linux" target="libfreetype.so.6" />
<dllmap dll="freetype6" os="freebsd" target="libfreetype.so.6" />
<dllmap dll="freetype6" os="freebsd" target="libfreetype.so.6" />
<dllmap dll="freetype6" os="freebsd" target="libfreetype.so.6" />
<dllmap dll="freetype6" os="freebsd" target="libfreetype.so.6" />
<dllmap dll="freetype6" os="osx" target="/Library/Frameworks/Mono.framework/Libraries/libfreetype.6.dylib" />
<dllmap dll="freetype6" os="freebsd" target="libfreetype.so.6" />
</configuration>