http2: remove spec coverage test

When run with -coverspec, tests track which portions of the
specification they cover and the TestSpecCoverage test produces
an error if any sections lack test coverage.

This is a lovely idea, and perhaps we should resurrect it at
some point, but there is currently exactly one coverage
annotation, dating back to the first commit of this package.

Change-Id: I5d2d8a1032b783d113ed0982f7e97a3bd1c07a33
Reviewed-on: https://go-review.googlesource.com/c/net/+/586243
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

Author: neild

http2/server_test.go

@@ -697,12 +697,6 @@ func TestServer(t *testing.T) {
 	})
 	defer st.Close()
 
-	covers("3.5", `
-		The server connection preface consists of a potentially empty
-		SETTINGS frame ([SETTINGS]) that MUST be the first frame the
-		server sends in the HTTP/2 connection.
-	`)
-
 	st.greet()
 	st.writeHeaders(HeadersFrameParam{
 		StreamID:      1, // clients send odd numbers