fs/ntfs: remove redundant variable idx

fs/ntfs: remove redundant variable idx

The variable idx is assigned a value and is never read.  The variable is
not used and is redundant, remove it.

Cleans up clang scan build warning:
warning: Although the value stored to 'idx' is used in the enclosing
expression, the value is never actually read from 'idx'
[deadcode.DeadStores]

Link: https://lkml.kernel.org/r/20220517093646.93628-2-colin.i.king@gmail.com
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Anton Altaparmakov <anton@tuxera.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>