YOUNGSTOWN — John Mayer is a man of many guitars and many genres. And there are the Deadheads, who (mostly) have embraced Mayer as part of Dead and Company for the past decade. But even in the quiet songs, Mayer found a slot for a guitar solo. However, the most Dead thing about Friday’s show was that Mayer felt no obligation to play many of his most popular songs. Even with two opening acts, a 90-minute headlining set by an artist in his prime and with a catalog as extensive as Mayer’s felt slight.