No it is not allow in a PFS game as Ghost Rider Replace/trade out Expert Trainer and
PFS Campaign Clarification wrote:
Ultimate Combat
Page 104 — A cavalier of 4th level or higher may take the Horse Master feat, ignoring the prerequisite of the expert trainer class feature. As per the cavalier's entry in Additional Resources, all cavaliers who would normally gain the expert trainer feature instead gain Skill Focus (Handle Animal). A cavalier who trades out expert trainer for another class feature as part of an archetype cannot take the Horse Master feat.
"PFS Additional Ressources wrote:
Ultimate Combat
A cavalier of 4th level or higher may take the Horse Master trait, ignoring the expert trainer class feature. A cavalier who trades this class feature for another as part of an archetype can not take the Horse Master feat.
