gopls/internal/test/marker: simplify seedCache file

Simplify the file used to seed the marker test cache, as suggested in CL
588940.

Change-Id: I421a3e013fcc17f2c6ab2ff5c269e6f360ca9d6e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/588942
Auto-Submit: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: findleyr

gopls/internal/test/marker/marker_test.go

@@ -296,17 +296,10 @@ func seedCache(t *testing.T, cache *cache.Cache) {
 	// The the doc string for details on how this seed was produced.
 	seed := `package p
 import (
-	"net/http"
-	"sort"
-	"go/types"
-	"testing"
-)
-
-var (
-	_ = http.Serve
-	_ = sort.Slice
-	_ types.Type
-	_ testing.T
+	_ "net/http"
+	_ "sort"
+	_ "go/types"
+	_ "testing"
 )
 `