all: remove PULL_REQUEST_TEMPLATE from .github

Update golang/go#18517

Change-Id: I76d928d5fcc5ed22beaffb86f0fa8fbf6d4ac3d7
Reviewed-on: https://go-review.googlesource.com/93035Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

.github/PULL_REQUEST_TEMPLATE

-Please do not send pull requests to the golang/* repositories.
-
-We do, however, take contributions gladly.
-
-See https://golang.org/doc/contribute.html
-
-Thanks!