MINOR: quic: Useless statement in quic_crypto_data_cpy()

This should fix Coverity CID 375057 in GH #1526 where a useless assignment
was detected.
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index 0793967..5aa1893 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -972,7 +972,6 @@
 			/* Increment the total size of this CRYPTO buffers by <to_copy>. */
 			qel->tx.crypto.sz += to_copy;
 			(*qcb)->sz += to_copy;
-			pos += to_copy;
 			len -= to_copy;
 			data += to_copy;
 		}