MLBEDSW-9912: Make sure weight buffering cost is cleared

 - Fixed a problem when the final scheduler cost contained wrong info
about Weight buffer when it should not. This caused the tensor
allocation to fail due to over allocation
 - The reason is that after optimizing the cascades, the final call to
ProposeScheduleBuffering decided that it would not need any buffering.
 - However, due to some missing code the weight buffering cost was not
cleared so memory size went over the limit
 - Added code to clear the weight buffering cost when it is not needed

Change-Id: I92db5c6199c99ca1ae4a21b62dfd17f2cae6ac9b
Signed-off-by: Johan Alfven <johan.alfven@arm.com>