ref-manual/classes.rst: remove .bbclass from section titles

This is unnecessary and allows to simplify the references
to such sections, most of which are removing the .bbclass extension
anyway, for example in :ref:`insane <ref-classes-insane>`.

(From yocto-docs rev: 44fbdb909c47553fecb7f9eb37f065a8768bc922)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Suggested-by: Quentin Schulz <foss+yocto@0leil.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>