mkvmuxerutil: remove unneeded const qualifier in cast

quiets -Wignored-qualifiers warning

Change-Id: Id5ec2ff69f84dd4c415972f2482b10b2049c5903
diff --git a/mkvmuxer/mkvmuxerutil.cc b/mkvmuxer/mkvmuxerutil.cc
index 7636a9f..6436817 100644
--- a/mkvmuxer/mkvmuxerutil.cc
+++ b/mkvmuxer/mkvmuxerutil.cc
@@ -508,7 +508,7 @@
   if (WriteUInt(writer, length))
     return false;
 
-  if (writer->Write(value, static_cast<const uint32>(length)))
+  if (writer->Write(value, static_cast<uint32>(length)))
     return false;
 
   return true;