Remove unnecesary drop() from ByteString1.writeToBuffer(ByteBuffer) (#25151)

I found a strange call to `drop()` in `ByteString1.writeToBuffer(ByteBuffer)`. `drop()` instantiates a new `ByteString1` which is immediately discarded to GC. Appears unnecessary.

Removing it makes the `ByteString1` implementation match the one `ByteString1C`.

akka-actor/src/main/scala/akka/util/ByteString.scala

@@ -330,7 +330,6 @@ object ByteString {
       val copyLength = Math.min(buffer.remaining, length)
       if (copyLength > 0) {
         buffer.put(bytes, startIndex, copyLength)
-        drop(copyLength)
       }
       copyLength
     }