Remove deprecation warning from ExtraInputs and ExtraOutputs properties

They cannot be deprecated until there is an alternative method to handle artificial dependencies.

See discussion in !633 (merged)

• [2018-03-15 17:15] Validation started with lhcb-gaudi-merge#456
• [2018-03-16 00:05] Validation started with lhcb-gaudi-head-py3#94
• [2018-03-16 00:07] Validation started with lhcb-tdr-test#90
• [2018-03-16 00:08] Validation started with lhcb-gaudi-head#1800
• [2018-03-16 08:03] Validation started with lhcb-gaudi-head#1801

added To Do label

mentioned in commit 942510e9

merged