account for deleted boards

[jepler]

Over in #5516 there was a build error about the removed board:

Adding docs/boards to build based on changed files
Building docs: False
Building boards:
Traceback (most recent call last):
  File "ci_set_matrix.py", line 131, in <module>
    main()
  adafruit_feather_esp32s2
  File "ci_set_matrix.py", line 127, in main
  adafruit_feather_esp32s2_nopsram
    set_boards_to_build(build_all)
  File "ci_set_matrix.py", line 91, in set_boards_to_build
    arch = PORT_TO_ARCH[board_to_port[board]]
KeyError: 'adafruit_feather_esp32s2_nopsram'

deleted files cause the related code to build (this is sensible! don't change it) but if what's deleted is a whole board you get an error like that.

[dhalbert]

Thanks for the quick fix!

[tannewt]

Thank you!