The repair for for btree_ptrs was saying one thing and doing another -
fortunately, that code can just be deleted.

Also, when we update a btree node pointer, we also have to update node
in memery, if it exists in the btree node cache - this fixes
bch2_check_fix_ptrs() to do that.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>