gateway: split olsr_delete_gateway_entry
gateway: split olsr_delete_gateway_entry

The olsr_delete_gateway_entry function now is a wrapper around
the olsr_delete_gateway_tree_entry function. The latter function
will be used by the cleanup function later on, to prevent needless
iterations on the gateway tree.

Signed-off-by: Ferry Huberts <66e11f9f5d965b69497c7e73d5e808f8b13c1218@pelagic.nl>

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