src/cmd/go/internal/load: remove PackageError.Hard

This field isn't actually used. The last usage was deleted in CL 518775
and even then it wasn't actually being set.

Change-Id: Ifbe9da9c2a6d81a40c136a60028902176f443588
Reviewed-on: https://go-review.googlesource.com/c/go/+/610255
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/internal/load/pkg.go

@@ -459,7 +459,6 @@ type PackageError struct {
 	Pos              string   // position of error
 	Err              error    // the error itself
 	IsImportCycle    bool     // the error is an import cycle
-	Hard             bool     // whether the error is soft or hard; soft errors are ignored in some places
 	alwaysPrintStack bool     // whether to always print the ImportStack
 }