CmkKpiStatCard: remove the unused good/bad delta coloring

DeltaSemantics let a consumer color the delta green or red depending on
whether an increase was an improvement or a regression, judged against
the metric's meaning. No consumer ever set anything but 'neutral' - the
delta already rendered without color, direction shown by the triangle
glyph alone. But the good/bad path was still live, exported API surface:
any future consumer could opt into coloring the delta, quietly breaking
that guarantee.

Removing DeltaSemantics and resolveDeltaColor() closes off that path,
making "the delta never carries color" a structural fact rather than
an incidental one. Updates DashboardContentNetworkFlowKpiStatCard.vue
and UclCmkKpiStatCard.vue, the two consumers.

CMK-38082

Change-Id: I57104e8376b026da5e5c990a558e92d28bc5ff8f