Don't worry about it, just use them, they'll clean themselves up. Think how much mud and grit MTB brakes see?
We have some clunker bikes at work that are used by all and sundry in all weather and conditions, they get washed about twice a year. My commuter sees little better.
If you must wash off with warm water, or buy some brake cleaner from the local motor factors (far cheaper than bike stuff) just dont get it in the wheel bearings. - Its handy to have if you ever get oil or lube overspray onto the disks and for cleaning up the calipers when you do a pad change.
9ne mechanic reckoned I glazed my brakes on my weekend bike, as I hardly used them. Yet the commuter travelled all winter picking up all sorts of muck and never even squeaked once.
Never really been troubled with that as it is pretty hilly around here. But did once, three weeks camping by the sea (Quibron) sorted it out where cleaning and faffing didn't, I reckon the salt and or sand abraded deglazed the pads.