Remove unused network_api.get_floating_ips_by_fixed_address()

Last usage was removed in 61e5b8e7da3b36db9a09f80d62ebf2e276bbe88b

Change-Id: I55626e7174b2380369b80fbced15093c2105aec1
Merge "Remove unused network_api.get_floating_ips_by_fixed_address()"