boards: arm: unify Nucleo-64 boards connectors image

At the moment there are two images for Nucleo-64 STM32 family board
connectors (img/nucleo_xxxxxx_connectors.png).

The images have the same canvas size (800x619) but different file
size in bytes and different color coding.

The first one (192K, 8-bit colormap) is used for

  * nucleo_f070rb
  * nucleo_f091rc
  * nucleo_f103rb
  * nucleo_l053r8
  * nucleo_l073rz

The second one (464K, 8-bit/color RGBA) is used for

  * nucleo_f030r8
  * nucleo_f302r8
  * nucleo_f334r8

Applying simultaneous black/white threshold to the images and
comparing them with imagemagick tools shows that the images
have no significant difference. Therefore we can use
nucleo_l073rz_connectors.png for nucleo_f030r8, nucleo_f302r8
and nucleo_f334r8.

Please see https://github.com/zephyrproject-rtos/zephyr/pull/15926


for details.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>