[Theme] Consider only one variant from the tchap design

[giomfo]

We defined in Tchap-ios a Style protocol (see) to handle 2 styles (variants) related to the light theme.

After discussion with the DINUM, we decided to consider only one variant as the actual light theme.
This will ease the rebase against Element-ios.

The retained variant is the variant2 (white header)

The aim here is to remove the use of the Style protocol in the Tchap screens. We will replace this style with the existing ThemeService call (see ThemeService.shared.theme in Element-ios code) . Only the light mode will be supported for the moment, it will be configured with the current variant2 of Tchap.