pallet-message-queue: Fix max message size calculation (#6205)

The max size of a message should not depend on the weight left in a
given execution context. Instead the max message size depends on the
service weights configured for the pallet. A message that may does not
fit into `on_idle` is not automatically overweight, because it may can
be executed successfully in `on_initialize` or in another block in
`on_idle` when there is more weight left.

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Co-authored-by: GitHub Action <[email protected]>
(cherry picked from commit 5d7181cd)