cmd/vet: lostcancel: suppress the check in the main.main function

When main.main returns, the process exits, so there's no need to cancel contexts.

This change was initially reviewed as
https://go-review.googlesource.com/c/go/+/106915/4
but somehow I messed up and committed patchset 5, which was
effectively empty.

Change-Id: Ic4250eb6563af9bc734e429aafc7081ca7d0e012
Reviewed-on: https://go-review.googlesource.com/c/148758


Reviewed-by: Alan Donovan <adonovan@google.com>