themes: remove various @extend with compound selectors

Merged Christoph Reiter requested to merge creiter/gtk:sass-no-extend-compound into gtk-3-24

They are no longer supported by sass and broken with libsass 3.6.3 (

This removes some of them by replacing them with a placeholder selector. This at least brings the resulting CSS size down a bit so gtk can be build again.

The remaining cases I don't know how to convert because I haven't found a way to reproduce the old output.

The CSS was generated with libsass 3.5.5.

See #2237 (closed)

Edited by Christoph Reiter

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