Leica LIF: do not set Channel.Color for RGB images (rebased onto dev_5_0)



This is the same as gh-1193 but rebased onto dev_5_0.

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The stored color for RGB images will usually (always?) be 0xffffffff, which causes the
displayed colors to be inaccurate (since the 'R' component will be shown as white).

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/12214 (QA 9185).  The ```Color``` attribute on ```Channel``` should not be set at all for this file; there should be no other change in behavior.