flash/nor/nrf5: remove is_erased setting and autoerase before write

flash/nor/nrf5: remove is_erased setting and autoerase before write

Cached flash erase state in sectors[].is_erased is not reliable as running
target can change the flash.

Autoerase was issued before flash write on condition is_erased != 1
Remove autoerase completely as it is a quite non-standard feature.

Change-Id: I19bef459e6afdc4c5fcaa2ccd194cf05be8a42b6
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4400
Tested-by: jenkins