Remove unneeded try..except in heartbeat
Remove unneeded try..except in heartbeat

as callback_url is now a required argument for this method,
there's no need for such check (presumably copy-pasted from
heartbeat-as-vendor-interface).

Besides, assigning a value to dictionary key never raises KeyError.

The now deprecated vendor heartbeat is unchanged.

Change-Id: Id1e1a15b2cbc5dfbae9655b0edbc30c0a0cf2c57

pshchelo/ironic,openstack/ironic,pshchelo/ironic,SauloAislan/ironic,NaohiroTamura/ironic,openstack/ironic,NaohiroTamura/ironic,SauloAislan/ironic