Remove block destruction being cancellable with "pick block"

A simple change, but I definitely think it would be for the better. It is common that the "pick block" key gets fat-fingered while destroying a block for some people. This is a nuisance, because it replaces the held item with the block being mined (if the player has it in their inventory). Since the held item is changed, the progress of the block's destruction gets reset. There isn't really a time where you'd want to pick a block while mining it, so I suggest that the block picking functionality should be "blocked" (ha) while mining.