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.c

@@ -14759,34 +14759,6 @@ int lfsr_traversal_rewind(lfs_t *lfs, lfsr_traversal_t *t) {
 
 /// Incremental gc operations ///
 
-#ifdef LFS_GC
-int lfsr_gc_setflags(lfs_t *lfs, uint32_t flags) {
-    // unknown gc flags?
-    LFS_ASSERT((flags & ~(
-            LFS_GC_MTREEONLY
-                | LFS_GC_MKCONSISTENT
-                | LFS_GC_LOOKAHEAD
-                | LFS_GC_COMPACT
-                | LFS_GC_CKMETA
-                | LFS_GC_CKDATA)) == 0);
-
-    // clobber any existing traversals
-    if (lfsr_omdir_isopen(lfs, &lfs->gc.t.o.o)) {
-        lfsr_omdir_close(lfs, &lfs->gc.t.o.o);
-    }
-
-    lfs->gc.flags = flags;
-    return 0;
-}
-#endif
-
-#ifdef LFS_GC
-int lfsr_gc_setsteps(lfs_t *lfs, lfs_soff_t steps) {
-    lfs->gc.steps = steps;
-    return 0;
-}
-#endif
-
 #ifdef LFS_GC
 // perform any pending janitorial work
 int lfsr_gc(lfs_t *lfs) {