boards: stm32: Remove lptim node when not used in tree

These boards enable lptim node, but don't propose 'power-states'
description, making it useless and not testable regarding configuration
of existing, in tree, PM tests.

Clean up this definition.

They will be put back along with 'power-states', when possible to test.


Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>