Remove incorrect statement about EFI environment variables being unsupported.

Our uefi support has included environment variable support for several years
now. Remove the bogus blanket statement saying we don't support them.

MFC After: 3 days
Approved by: re@ (gjb)
(cherry picked from commit ab77cc9e7bf6d0c2d862dfd514539b81ae248dfd)
(cherry picked from commit 78050d4966c5a9c8e953cab2a286450238d51e4f)