Closes https://github.com/zed-industries/zed/issues/25413
As the issue points out well, themes do not need to alter any in-memory
state on load: that is done via settings file load.
Originally, it was introduced in
https://github.com/zed-industries/zed/pull/4064 and
https://github.com/zed-industries/zed/pull/24857 had restored that
behavior, which seems wrong to do.
Apart from removing that part, removes unnecessary methods and
emphasizes that in-memory state is the Buffer/UI size — no need to add
`Adjusted` there as the settings file presence is already enough.
Release Notes:
- Fixed theme selector resetting the buffer size