scripts: sphinx-pre-install: move get_system_release()

scripts: sphinx-pre-install: move get_system_release()

The code at get_system_release() is actually a helper function,
independent from the actual Sphinx verification checker. Move
it to MissingCheckers class, where other checkers are present.

With that, the entire distro-specific handler logic, with
all its complexity is confined at SphinxDependencyChecker
class.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/4b42a85bbb6575bb34a58cf66019038c4afa1d5b.1754992972.git.mchehab+huawei@kernel.org