scripts: genpinctrl: remove pwm variant for timer channels

STM32 timers are multi-purpose peripherals ranging from PWM, Input
capture, encoder, HALL sensor... In all series except F1 the timer pins
will always be configured in the same mode, i.e. alternate. So using a
variant for each functionality is not necessary. When working with F1
series mode is still required because the pin may operate in either
alternate or input mode depending on the assigned functionality.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>