all: Fix some linter errors (#5499)

I'm working through linter complaints, these are some fixes. Broad
categories:

1) Ignore errors where we can ignore errors: add "_ = ..." construct.
you can argue that this is annoying noise, but apart from silencing the
linter it *does* serve the purpose of highlighting that an error is
being ignored. I think this is OK, because the linter highlighted some
error cases I wasn't aware of (starting CPU profiles, for example).

2) Untyped constants where we though we had set the type.

3) A real bug where we ineffectually assigned to a shadowed err.

4) Some dead code removed.

There'll be more of these, because not all packages are fixed, but the
diff was already large enough.

cmd/syncthing/gui_test.go

@@ -114,13 +114,13 @@ func TestAssetsDir(t *testing.T) {
 	// The asset map contains compressed assets, so create a couple of gzip compressed assets here.
 	buf := new(bytes.Buffer)
 	gw := gzip.NewWriter(buf)
-	gw.Write([]byte("default"))
+	_, _ = gw.Write([]byte("default"))
 	gw.Close()
 	def := buf.Bytes()
 
 	buf = new(bytes.Buffer)
 	gw = gzip.NewWriter(buf)
-	gw.Write([]byte("foo"))
+	_, _ = gw.Write([]byte("foo"))
 	gw.Close()
 	foo := buf.Bytes()