imsm: Do not require MDADM_EXPERIMENTAL flag anymore

imsm: Do not require MDADM_EXPERIMENTAL flag anymore

Grow feature for IMSM metadata is currently fully supported and tested.
Reshape operation is not in experimental state anymore, so usage of this
flag is unnecessary.

Do not require MDADM_EXPERIMENTAL flag and remove obsolete information
from manual.

Signed-off-by: Michal Zylowski <michal.zylowski@intel.com>
Acked-by: Mariusz Dabrowski <mariusz.dabrowski@intel.com>
Acked-by: Roman Sobanski <roman.sobanski@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>