Fixes usage of mixed enum flags.

[dkosmari]

• Undo warning suppression introduced in 9c34c33.
• ImGui's enums are static_cast to eliminate the warning.
• ImPlot's enums are extended with _NoLegend and _NoFit so there's never a need to mix them.

[brenocq]

I'm planning to keep the warning suppression since this was the solution implemented in ImGui, when ImGui implements a different solution I'm more than happy to implement the same here.

I can see the benefit of extending ImPlot's enums with _NoLegend/_NoFit, but this makes it a bit harder to maintain when we add new item flags. For now I'll keep as is since the suppression was the ImGui solution.

[dkosmari]

You have a _None flag repeated for every single one of these enums.

You already created maintenance hell because adding a new "generic" enum requires making sure it doesn't clash with any of the other "specialized" flags.

And in case somebody forgets to add the _NoLegend and _NoFit flags to a brand new enum, nothing will break, the user can still use the generic version (with an explicit cast if compiling with warnings.)