[Collator Selection] Fix weight refund for `set_candidacy_bond` (#3643)

Fixes #3642 

This PR implements the weight refund of
`pallet_collator_selection::set_candidacy_bond` to account for no
iterations when the bond is decreased.

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Signed-off-by: georgepisaltu <george.pisaltu@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>