common: remove {Left, Right}PadString (#168)

Fixes #134

Those functions are unused in the whole Tendermint Github
organization plus they were unnecessariy verbose and could
have been concisely replaced with

```go
func RightPadString(s string, totalLength uint) string {
       return fmt.Sprintf("% *s", totalLength, s)
}

func LeftPadString(s string, totalLength uint) string {
       return fmt.Sprintf("% -*s", totalLength, s)
}
```

delete them anyways

common/string.go

@@ -15,24 +15,6 @@ var Fmt = func(format string, a ...interface{}) string {
 	}
 }
 
-// RightPadString adds spaces to the right of a string to make it length totalLength
-func RightPadString(s string, totalLength int) string {
-	remaining := totalLength - len(s)
-	if remaining > 0 {
-		s = s + strings.Repeat(" ", remaining)
-	}
-	return s
-}
-
-// LeftPadString adds spaces to the left of a string to make it length totalLength
-func LeftPadString(s string, totalLength int) string {
-	remaining := totalLength - len(s)
-	if remaining > 0 {
-		s = strings.Repeat(" ", remaining) + s
-	}
-	return s
-}
-
 // IsHex returns true for non-empty hex-string prefixed with "0x"
 func IsHex(s string) bool {
 	if len(s) > 2 && strings.EqualFold(s[:2], "0x") {