boards: remove pinmux as supported feature

The tag that marks pinmux as supported is not used by any test or
sample, therefore this doesn't seem needed anymore.

- remove from 3 mec1x boards, they still use pinmux,
  but as the tag is neither used in tests and pinmux
  deprecated anyway no need for it.
- remove from 5 esp32 boards, as they now use pinctrl.
- remove from 2 lpc boards, as they now use pinctrl.
- remove from 1 stm32 board, as it now uses pinctrl.
- remove from 1 npcx board, as it now uses pinctrl.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>