cmd/link: remove name expansion logic

Now both the compiler and the assembler require the -p flag and
emit full package path in symbol names, we no longer need to do
the name expansion in the linker. Delete it.

Change-Id: I771d4d97987a0a17414881b52806d600ef4cc351
Reviewed-on: https://go-review.googlesource.com/c/go/+/404300
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>

src/cmd/link/internal/loader/loader_test.go

@@ -198,7 +198,7 @@ func TestAddMaterializedSymbol(t *testing.T) {
 	}
 
 	// Nameless symbol should still be nameless.
-	es3name := ldr.RawSymName(es3)
+	es3name := ldr.SymName(es3)
 	if "" != es3name {
 		t.Errorf("expected es3 name of '', got '%s'", es3name)
 	}