maint: remove various FIXME comments

maint: remove various FIXME comments

* src/extent-scan.c (extent_need_sync): Remove the FIXME comment about
removing the work around.  As discussed in [1], the needed FIEMAP fix
never made it into the Linux kernel.

* src/realpath.c (longopts): Remove the FIXME comment about deprecating
the --strip option as it's a valid alias for --no-symlinks option.
Also discussed in [1].

[1] http://lists.gnu.org/archive/html/coreutils/2013-11/msg00103.html