bcachefs: Kill bch2_ec_mem_alloc()

bcachefs: Kill bch2_ec_mem_alloc()

bch2_ec_mem_alloc() was only used by GC, and there's no real need to
preallocate the stripes radix tree since we can cope fine with memory
allocation failure when we use the radix tree. This deletes a fair bit
of code, and it's also needed for the upcoming patch because
bch2_btree_iter_peek_prev() won't be working before journal replay
completes (and using it was incorrect previously, as well).

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>