Remove STM32HXX in CMakeList.txt (#76)

<!--- Title -->
Remove STM32HXX in CMakeList.txt

Description
-----------
As #75 mentioned, STM32HXX is listed in CMakeList.txt but no portable
layer for it. We don't really support STM32HXX series at this moment.
Remove it in CMakeList to avoid confusion.

Test Steps
-----------
<!-- Describe the steps to reproduce. -->

Checklist:
----------
<!--- Go over all the following points, and put an `x` in all the boxes
that apply. -->
<!--- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
- [ ] I have tested my changes. No regression in existing tests.
- [ ] I have modified and/or added unit-tests to cover the code changes
in this Pull Request.

Related Issue
-----------
<!-- If any, please provide issue ID. -->

By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice.

Author: ActoryOu

CMakeLists.txt

@@ -38,7 +38,6 @@ set(FREERTOS_PLUS_FAT_PORT_LIST
     POSIX
     STM32F4XX
     STM32F7XX
-    STM32HXX
     ZYNQ
     ZYNQ_2019_3
 )