The main function of bch2_sort_repack_merge() was to call .key_normalize
on every key, which drops stale (cached) pointers - it hasn't actually
merged extents in quite some time.

But bch2_gc_gens() now works on individual keys - we used to gc old gens
by rewriting entire btree nodes. With that gone, there's no need for
internal btree code to be calling .key_normalize anymore.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>