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Lay a soaking wet (just water) towel on it for tweenty minutes, then wash with soap and water. Should come right off. If you have a windshield, do the same thing, just use the wet towel with no soap to clean it.
I use bug and tar remover. I spray it on the bike anywhere i see gunk and let it sit while i fill the bucket with soap and water and get the towels and wash mit ready. It comes right off with little effort and no residue after you rinse it off.