Gambling revenues have dropped at Encore Boston Harbor even as they’ve rebounded at Massachusetts’ two other casinos. The state’s Gaming Commission reported Tuesday that the Everett resort generated less than $49 million in gross gambling revenues in January, down from $54 million in December. Revenues at MGM Springfield, meanwhile, rose to nearly $21 million in January, up from about $19 million in December. And Plainridge Park in Plainville generated $11 million from gambling in January, up from $10 million in December.