gin.settings.third_party_settings config schema is wrong

Problem/Motivation

gin.settings.third_party_settings is incorrectly overwriting core's theme_settings definition.

It is basically the same bug as #3402885 - olivero.settings config schema is wrong 🐛 olivero.settings config schema is wrong Active copied 1:1 into Gin.

But it also has the nasty side effect of breaking theme third party settings for every other module except shortcut.

Remaining tasks

Wait for #3087036 - third_party_settings in THEME.settings.yml cannot be dependency managed 🐛 third_party_settings in THEME.settings.yml cannot be dependency managed Needs work .
Then remove the incorrect schema definition.