remove assetPrefix in normalizeRoute fn

[tscanlin]

This change makes it so you can easily serve multiple static next apps from subfolders on the same domain using the already available assetPrefix option.

Without this change the file that's requested is like:
/${assetPrefix}/_next/c0bae9f4-37d0-4f02-8a91-08fe02d6fbfb/page/${assetPrefix}/index.js

instead of:
/${assetPrefix}/_next/c0bae9f4-37d0-4f02-8a91-08fe02d6fbfb/page/index.js

like you'd want.

[arunoda]

Serving Next.js apps from multiple directories is not a goal of Next.js at the moment.
So, I can't take this in.

[tscanlin]

FWIW it seems that this fix solved the issue for multiple next apps on one domain: #257

It would be awesome if we could get this landed.

[wesbos]

+1 for this - I'm trying to run Next though an express app to power my admin console.

[gcpantazis]

#2186 would supercede this by making the _next directory directly configurable.

assetPrefix is more meant for CDN, if memory serves?

[dvakatsiienko]

We are following the same approach with different next apps in nested pathes on the same domain. We need this functionality also!