flash/nor: Do not update 'is_erased'

flash/nor: Do not update 'is_erased'

Remove useless setting of the 'is_erased' flag after a flash
modification. Also, remove code that erroneously depends on
this flag.

Keep the flag for now since it is still used by the
erase_check() functionality.

See commit 07da3b3913dcb6b65564f4fe67c2f26032f180c0

Change-Id: Ia046905bc9e0cb2572145f19251c3e10c4e2d553
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: http://openocd.zylin.com/6279
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>