fix(ui5-avatar): remove redundant placeholder variables from horizon themes

[kgogov]

Remove --ui5-avatar-placeholder and --ui5-avatar-placeholder-color variables from horizon theme variants as they duplicate the base theme values.

The base Avatar-parameters.css already defines these variables correctly using

• --sapContent_ImagePlaceholderBackground
• --sapContent_ImagePlaceholderForegroundColor

as per design specification.

Fixes: #11750