fwk: Always zero list nodes when they are removed

This patch ensures that list nodes are always cleared after being
removed from a list. This ensures that functions operating on lists do
not have to manually zero these fields if they need to move the node to
another list.

Change-Id: I2c9b6e135a85ca8b8d1845db1df1c3004c1b49c9
Signed-off-by: Chris Kay <chris.kay@arm.com>