gc: Dropped lfsr_gc_setflags/setsteps

Now that you can provide gc_flags/gc_steps in lfs_config, I think it's a
bit more clear that _mutating_ the flags/steps is a niche feature, and
not worth implementing/testing.

It raises the question why not have a similar lfsr_setflags or
lfsr_file_setflags, and the answer there is it would be a pain-in-the-
ass to make sure all possible corner cases are covered.

It actually already was a pain-in-the-ass to test lfsr_gcsetflags/
setsteps... but just because we already did the work is not a good
reason for keeping complexity around.

---

Note that most of the use cases for lfsr_gc_setflags/setsteps can be
covered by either remounting the filesystem or through the
lfsr_traversal_t APIs directly.

The end result is a bit of code savings when incremental gc is enabled:

                   code          stack          ctx
  default before: 37796           2608          620
  default after:  37796 (+0.0%)   2608 (+0.0%)  620 (+0.0%)

  gc before:      37944           2608          768
  gc after        37896 (-0.1%)   2608 (+0.0%)  768 (+0.0%)

lfs.h

@@ -1270,27 +1270,6 @@ int lfsr_traversal_rewind(lfs_t *lfs, lfsr_traversal_t *t);
 int lfsr_gc(lfs_t *lfs);
 #endif
 
-#ifdef LFS_GC
-// Sets the gc flags.
-//
-// Returns a negative error code on failure.
-int lfsr_gc_setflags(lfs_t *lfs, uint32_t flags);
-#endif
-
-#ifdef LFS_GC
-// Sets the number of gc steps per lfsr_gc call, with each step being
-// ~1 block of work.
-//
-// More steps per call will make more progress if interleaved with
-// other filesystem operations, but may also introduce more latency.
-// steps=1 will do the minimum amount of work to make progress, and
-// steps=-1 will not return until all pending janitorial work has
-// been completed.
-//
-// Returns a negative error code on failure.
-int lfsr_gc_setsteps(lfs_t *lfs, lfs_soff_t steps);
-#endif
-
 
 /// Filesystem-level filesystem operations