Remove %% from time formats.

It's not actually possible to input these values, so it's hard to write
a test for it.  Best to just remove it until it's needed.

Closes #37
diff --git a/times.go b/times.go
index 085d2f1..49906b3 100644
--- a/times.go
+++ b/times.go
@@ -76,7 +76,6 @@
 	for _, ch := range mag.format {
 		if escaped {
 			switch ch {
-			case '%': // TODO:  coverage missing
 			case 's':
 				args = append(args, lbl)
 			case 'd':