TAJO-1207: Remove redundant code in Bytes.java in tajo-common

in Bytes.java
padHead and padTail
Don't need to set array to 0.
because java alloc byte array and initialize them as 0.
```java
byte [] padding = new byte[length];
//so below is redundant block
for (int i = 0; i < length; i++)
{ padding[i] = 0; }
return add(padding,a);
```