meson: remove yaml configuration

In the long term interest of the project, remove the option to use
yaml configuration.

It's been causing too many issues and inconsistencies regarding led
names and the limitations of the yaml configuration format. It also
makes it harder to further develop led-manager as all configuration
related features have to be developed and tested twice.
[1],[2],[3],[4]

It's expected to be simpler for users to only have one way to configure
the led-manager and the run-time json configuration is more flexible
for development.

Looking forward, the json configuration will also more easily migrate to
EM configuration, allowing to implement dynamic configuration for
led-manager in the future.

The tradeoff is that there is a small delay for the led-manager to parse
its configuration.

The existing yaml configs have been ported [5] on a best-effort basis.
It will rely on platform maintainers to chip in and help since yocto
layers can be difficult to patch and understand their structure.

Tested: Unit tests pass. Help welcome to test and review [5]

References:

[1] https://gerrit.openbmc.org/c/openbmc/phosphor-led-sysfs/+/57455/comments/b3327528_005dec78
[2] https://gerrit.openbmc.org/c/openbmc/phosphor-led-sysfs/+/77238
[3] https://gerrit.openbmc.org/c/openbmc/phosphor-led-manager/+/74349
[4] https://gerrit.openbmc.org/c/openbmc/phosphor-led-manager/+/77230
[5] https://gerrit.openbmc.org/c/openbmc/openbmc/+/78131

Change-Id: Ic4047d192a6086bcbc5a483611024442779b1e89
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>