Remove redundant header files in STM port

[flom84]

Hi everyone,

Working with the SMT32 ports, I have found a few header files being included that don't seem to have any role in the build. I made this change to remove them, and tested it on all the stm builds.

If you confirm that these includes are redundant, the change could be made for the other ports as well.

[jepler]

This is fine if it comes up green on CI, it has a very low probability of creating any regressions. However, I do have one note for the future.

[dhalbert]

@tannewt has talked about https://include-what-you-use.org/, which automatically determines the proper header files to include. It's based on clang, not sure how/if we could use it.

[flom84]

@dhalbert thanks for the tool suggestion. I was not aware of it. In my case I have used https://github.com/rizsotto/Bear for generating the compilation database for clang tooling. Ex: bear --append -- make BOARD=circuitplayground_express V=2 -j4
And then Gvim Editor + https://github.com/ycm-core/YouCompleteMe (clang tool support)....and with this I can see what headers can be removed...indeed it's manual work.

[microdev1]

Thanks! @flom84, LGTM.