Remove mbed OS and mbed 2 version macros. As master can break anytime (should
not break but can happen and shall be expected that there are changes that might affect the bleeding edge developers), there is no remedy for protecting
the sw via macros (either using some special versions, etc). If it is not
defined, means it is developed version of mbed OS/mbed 2 and it is up to a user
to track changes.

Any release of mbed OS 5 and mbed 2 contains these version macros. Thus won't break anything.