Remove the unreachable checking code

Motivation:
'length2 == 0' is not reachable because length1 and length2 are same at this point.

Motification:
Removed 'length2 == 0'.

Result:
Cleaner code.

common/src/main/java/io/netty/util/AsciiString.java

@@ -959,9 +959,7 @@ public boolean contentEquals(CharSequence cs) {
         int length2 = cs.length();
         if (length1 != length2) {
             return false;
-        }
-
-        if (length1 == 0 && length2 == 0) {
+        } else if (length1 == 0) {
             return true; // since both are empty strings
         }