arm: nucleo_f103rb: Do not enable PWM by default

The test "net/utils" fails to build on nucleo_f103rb
due to "region `SRAM' overflowed by 8 bytes.

Disabling PWM at the board level to build the test.

Moreover, according to issue #6858, we will only configure
board valuable HW and connectors on board level and the
user should enable what is needed on application level.

Reported-by: Yannis Damigos <giannis.damigos@gmail.com>
Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>