Remove next-to-next-server babel plugin

[ijjk]

After discussion it was decided instead of rewriting next/config and next/head imports to next-server/config and next-server/head we should just allow importing them as next/config and next/head

Fixes: #6187

[chrisbolin]

thank you @ijjk! this fixes an ugly issue in our jest tests where we have to mock next-server/dynamic instead of next/server (it was also tough to debug the issue when we upgraded to next@8 😂)

really appreciate it!