coremqtt-agent: Improve subscription_manager.c

Previously, removeSubscription did not produce any output, nor return a
value if it failed to remove a subscription (e.g. if the subscription
did not exist).
This commit makes removeSubscription return true if successful,
and false otherwise. This commit also documents this change.

Signed-off-by: Reuben Cartwright <Reuben.Cartwright@arm.com>