MAINTAINERS: drop an old reference to stm32 pwm timers doc

The DT files for pwm were merged and converted to json.
The new reference is already at the maintainers file, so
just drop the obsoleted one.

Fixes: 56fb34d8 ("dt-bindings: mfd: Convert stm32 timers bindings to json-schema")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>