treewide: remove unnessary "coding" header

treewide: remove unnessary "coding" header

The "-*- coding: utf-8 -*-" header was needed in Python 2,
but in Python 3 UTF-8 is already the default encoding of
source files.

It is even less necessary in .css files that do not have UTF-8
sequences at all.

Suggested-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20250929154938.594389-4-pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>