QRingBuffer: remove QByteArray::resize()-shrinks work-around

QByteArray::resize() is now documented to not shrink capacity(), so
remove the extra reserve(0) work-around.

Change-Id: I3d5d944b9f01f8f84b31599548773198903aad4c
Reviewed-by: Alex Trotsenko <alex1973tr@gmail.com>

src/corelib/tools/qringbuffer.cpp

@@ -82,7 +82,6 @@ QByteArray QRingChunk::toByteArray() &&
             headOffset = 0;
         }
 
-        chunk.reserve(0); // avoid that resizing needlessly reallocates
         chunk.resize(tailOffset);
     }