Remove the JsonSerializable interface from \OC\Files\FileInfo

Needed to make it compatible with php 5.3.

Afaik the only time this interface was needed was in the gallery app which has been fixed with https://github.com/owncloud/apps/commit/c2075766eeb2f6e6d56340e756964c04d0fb0fb3

cc @DeepDiver1975 @PVince81