closes #155 (closed) as it removes pending membership
closes #156 (closed) (not entirely, the rest will be done in a separate MR on smith wot)
closes #136 (closed) (it does not link instanciable pallets but fixes the test that were added to show the unwanted behavior)
remove request_membership call
remove PendingMembership storage item
remove PendingMembershipsExpireOn storage item
remove validate_identity call from identity pallet which is redundant with claim_membership of membership pallet
