Dropped fragmenting blocks > 1 fragment

So we now keep blocks around until they can be replaced with a single
fragment. This is simpler, cheaper, and reduces the number of commits
needed to graft (though note arbitrary range removals still keep this
unbounded).

---

So, this is a delicate tradeoff.

On one hand, not fully fragmenting blocks risks keeping around bptrs
containing very little data, depending on fragment_size.

On the other hand:

- It's expensive, and disk utilization during random _deletes_ is not
  the biggest of concerns.

  Note our crystallization algorithm should still clean up partial
  blocks _eventually_, so this doesn't really impact random writes.
  The main concerns are lfs3_file_truncate/fruncate, and in the future
  collapserange/punchhole.

- Fragmenting bptrs introduces more commits, which have their own
  prog/erase cost, and it's unclear how this impacts logging operations.

  There's no point in fragmenting blocks at the head of a log if we're
  going to fruncate them eventually.

I figure lets err on minimizing complexity/code size for now, and if
this turns out to be a mistake, we can always revert or introduce
fragmenting >1 fragment blocks as an optional feature in the future.

---

Saves a big chunk of code, stack, and even some ctx (no more
fragment_thresh):

           code          stack          ctx
  before: 37504           2448          656
  after:  37024 (-1.3%)   2416 (-1.3%)  652 (-0.6%)

runners/bench_runner.h

@@ -120,7 +120,6 @@ void bench_permutation(size_t i, uint32_t *buffer, size_t size);
     BENCH_DEFINE(INLINE_SIZE,        BLOCK_SIZE/4                           ) \
     BENCH_DEFINE(FRAGMENT_SIZE,      LFS3_MIN(BLOCK_SIZE/8, 512)            ) \
     BENCH_DEFINE(CRYSTAL_THRESH,     BLOCK_SIZE/8                           ) \
-    BENCH_DEFINE(FRAGMENT_THRESH,    -1                                     ) \
     BENCH_DEFINE(ERASE_VALUE,        0xff                                   ) \
     BENCH_DEFINE(ERASE_CYCLES,       0                                      ) \
     BENCH_DEFINE(BADBLOCK_BEHAVIOR,  LFS3_EMUBD_BADBLOCK_PROGERROR          ) \
@@ -149,8 +148,7 @@ void bench_permutation(size_t i, uint32_t *buffer, size_t size);
     .gc_compact_thresh  = GC_COMPACT_THRESH,    \
     .inline_size        = INLINE_SIZE,          \
     .fragment_size      = FRAGMENT_SIZE,        \
-    .crystal_thresh     = CRYSTAL_THRESH,       \
-    .fragment_thresh    = FRAGMENT_THRESH,
+    .crystal_thresh     = CRYSTAL_THRESH,
 
 #ifdef LFS3_GC
 #define BENCH_GC_CFG                            \