Fontconfig: simplify font caching process.

Fontconfig: simplify font caching process.

- Remove FcConfigGetCache(), since it returns NULL in all modern
  Fontconfig versions.

- Remove obsolete code.  Fontconfig automatically sets up rescanning
  intervals for font caches and does not use per-user cache files
  anymore.

- Use FcInitBringUptoDate(), which takes care of all caching issues
  behind the scenes.  For example, it will automatically call
  FcConfigBuildFonts() and FcConfigSetCurrent(), if necessary.