radix_tree_iter_resume: Fix out of bounds error

The address sanitizer occasionally finds an out of bounds error while
running the test-suite.  It turned out to be a read of the pointer
immediately next to the tree root, but this out of bounds error could
have occurred elsewhere.  This happens because radix_tree_iter_resume()
dereferences 'slot' before checking whether we've come to the end of
the chunk.  We can just delete this line; the value was never used.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>