time: delete unused buffer.WriteByte method

R=golang-dev, bradfitz, r, rsc
CC=golang-dev
https://golang.org/cl/5539056

src/pkg/time/format.go

@@ -335,10 +335,6 @@ func (b *buffer) WriteString(s string) {
 	*b = append(*b, s...)
 }
 
-func (b *buffer) WriteByte(c byte) {
-	*b = append(*b, c)
-}
-
 func (b *buffer) String() string {
 	return string([]byte(*b))
 }