misc/linkcheck: remove unused tool

Fixes #72953

Change-Id: I8a0c8da3f8309841147412a078bc82095a93c5b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/659275
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: seankhliao