fix: ElvUI skin now respects user Appearance settings

[Xerrion]

The ElvUI skin was unconditionally overriding font sizes, font outline, and background color/alpha set by the user in the Appearance tab.

Changes

• ElvUI provides only the font FACE (typeface) -- user font sizes, outline, and secondary font size from Appearance are respected
• Background color and alpha are never overridden by the skin -- user Appearance settings are the final word
• Border colors from ElvUI still apply when quality border is off (unchanged)
• Removed unused GetBackdropColor, GetBackdropTexture, and GetFontSize functions