Avoid unconditionally storsing input as runes

When --with-nth is used, fzf used to preprocess each line and store the
result as rune array, which was wasteful if the line only contains ascii
characters.

src/util/chars.go

@@ -157,46 +157,3 @@ func (chars *Chars) CopyRunes(dest []rune) {
 	}
 	return
 }
-
-func (chars *Chars) Slice(b int, e int) Chars {
-	if runes := chars.optionalRunes(); runes != nil {
-		return RunesToChars(runes[b:e])
-	}
-	return Chars{slice: chars.slice[b:e], inBytes: true}
-}
-
-func (chars *Chars) Split(delimiter string) []Chars {
-	delim := []rune(delimiter)
-	numChars := chars.Length()
-	numDelim := len(delim)
-	begin := 0
-	ret := make([]Chars, 0, 1)
-
-	for index := 0; index < numChars; {
-		if index+numDelim <= numChars {
-			match := true
-			for off, d := range delim {
-				if chars.Get(index+off) != d {
-					match = false
-					break
-				}
-			}
-			// Found the delimiter
-			if match {
-				incr := Max(numDelim, 1)
-				ret = append(ret, chars.Slice(begin, index+incr))
-				index += incr
-				begin = index
-				continue
-			}
-		} else {
-			// Impossible to find the delimiter in the remaining substring
-			break
-		}
-		index++
-	}
-	if begin < numChars || len(ret) == 0 {
-		ret = append(ret, chars.Slice(begin, numChars))
-	}
-	return ret
-}