qemu-img: remove unused variable

qemu-img: remove unused variable

Variable block_count used in img_dd function is only incremented but never read.
This causes 'Unused but set variable' warning on Clang 15.0.1 compiler.

Removing the variable to prevent the warning.

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <e86d5b57f9d13bde995c616a533b876f1fb8a527.1668009030.git.mrezanin@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit 0f48c47c679bc29fceb3a67283ee3b78175524bf)