[Coroutines] Remove unused variable [NFC]

gcc warned about it:
 ../lib/Transforms/Coroutines/CoroFrame.cpp:2785:15: warning: unused variable 'MD' [-Wunused-variable]
  2785 |   if (MDNode *MD = AI->getMetadata(LLVMContext::MD_coro_outside_frame))
       |               ^~

Fix the warning by removing the unused variable and change the call
from getMetadata to hasMetadata.