Revert "cpu/cc2538: sbrk() checks if the requested memory is really available."

This reverts commit 81dea364a5b372ad1625181eff710c095420735c.
malloc: check if the requested memory is really available
resurrect malloc test
Merge pull request #1782 from benpicco/fix_malloc

check if the requested memory is really available in _sbrk_r