titlebar_color set to `system` instead of a custom theme color

In issue #23 people agreed to use a custom theme color for the titlebar color.
A few months later PR #25 [reversed](https://github.com/catppuccin/kitty/pull/25/files?diff=split&w=0#diff-96fc96b9baa15202565247ece114d0c748c3b9a712a6de1e4a72992a6c039ac4R30-R31) this change. This change looks unintentional to me or am I missing something?

If not intentional, I think we should go a step further and remove the `{macos,wayland}-titlebar-color` from the themes altogether.
Users can simply set `{macos,wayland}-titlebar-color` to `background` in the main config file to accomplish the same thing, so there should be no need to hardcode specific colors in the themes. This matches what some (most?) other themes do and also makes it simpler for the user to change this setting if desired.

Start with the linked PR #25 diff and the theme definitions containing macos-titlebar-color and wayland-titlebar-color. Compare the intended behavior with issue #23 and the other themes; done means the titlebar settings consistently follow the agreed custom-color approach and the proposed redundant theme entries are addressed.

Written by the indexing model from the issue text.