cmd/cover: remove go.mod from testdata subdir

cmd/cover: remove go.mod from testdata subdir

Remove a superfluous go.mod file in one of the testdata subdirs; test
runs ok without it, no need for it to be there (can confuse tooling).

Change-Id: I3c43dd8ca557fdd32ce2f84cdb2427326a2dd35e
Reviewed-on: https://go-review.googlesource.com/c/go/+/461945
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>