docs(contribute): remove note about using elevated prompt on win

Originally added in 485f1040990508441cebf4cddab83a126e65355f, however
there is no note about why this is needed. I've just done a clean clone
and build on a Windows 8.1 machine using a non-elevated PowerShell prompt,
without any issues.

Leaving this in is equivalent to asking people to use sudo on *nix without
any justification.

If the message is reintroduced, it should have a reason included.