[release] remove docs/nightly directory

The directory was removed from the master, and
the directory deletion commit was lost during merge.

Change-Id: I484221cb1bf7308d7330a078fe24b8c6a25d4fc2
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/245482
Reviewed-by: Rebecca Stambler <[email protected]>

Author: hyangah