cmsis: remove math header file

It was removed in 6.0 (see reference below), and it was reintroduced when we updated
cmsis from the upstream. We missed to remove the commit adding the file in the cmsis
importer. This fixes it and the file should not be introduced again.

Fixes ARMmbed#13823

Already removed in 6.0: ARMmbed#12055

Author: 0xc0170