MLBEDSW-7487: Updated implementation for the Mean op

- Latest reference has changed implementation for the Mean op
and now only contain one variant.
- Updated Vela implementation to match reference. The full sum
is first calculated and then divided by the numbers of elements.
- Removed the avg pool variant and test case.
- Updated SUPPORTED_OPS.md

Change-Id: I4275e36e3697fa837f119f2cefd7c0ff94231605
Signed-off-by: Johan Alfven <johan.alfven@arm.com>