btrfs-tree: drop BtrfsFsTreeFetcher and clean up class comments

BtrfsFsTreeFetcher was used for early versions of the extent scanner, but
neither subvol nor extent scan now needs an object that is both persistent
and configured to access only one subvol.  BtrfsExtentDataFetcher does
the same thing in that case.

Clarify the comments on what the remaining classes do, so that
BtrfsFsTreeFetcher doesn't get inadvertently reinvented in the future.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/btrfs-tree.cc

@@ -662,14 +662,6 @@ namespace crucible {
 		type(BTRFS_EXTENT_DATA_KEY);
 	}
 
-	BtrfsFsTreeFetcher::BtrfsFsTreeFetcher(const Fd &new_fd, uint64_t subvol) :
-		BtrfsTreeObjectFetcher(new_fd)
-	{
-		tree(subvol);
-		type(BTRFS_EXTENT_DATA_KEY);
-		scale_size(1);
-	}
-
 	BtrfsInodeFetcher::BtrfsInodeFetcher(const Fd &fd) :
 		BtrfsTreeObjectFetcher(fd)
 	{