remove unnecessary [GoBoardRegion hasPoint:]

this method is not necessary, instead one can compare the GoBoardRegion object
with the [GoPoint region] property; more importantly, the object comparison is more
efficient than the [GoBoardRegion hasPoint:] test, so much so that loading a large
game becomes significantly faster on older devices (cf. profiling information in issue
#113 for iPhone 3GS and the ear-reddening game).