block: Remove unused assignment (fixes warning from clang)

blockdev.c:1929:13: warning: Value stored to 'ret' is never read
            ret = 0;
            ^     ~

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

blockdev.c

diff --git a/blockdev.c b/blockdev.c
index 8aa66a9..8c83f6f 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1926,7 +1926,6 @@
     } else {
         switch (mode) {
         case NEW_IMAGE_MODE_EXISTING:
-            ret = 0;
             break;
         case NEW_IMAGE_MODE_ABSOLUTE_PATHS:
             /* create new image with backing file */