[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.

Author: mikaelholmen

llvm/lib/Transforms/Coroutines/CoroFrame.cpp

@@ -2782,7 +2782,7 @@ static void collectFrameAlloca(AllocaInst *AI, coro::Shape &Shape,
 
   // The __coro_gro alloca should outlive the promise, make sure we
   // keep it outside the frame.
-  if (MDNode *MD = AI->getMetadata(LLVMContext::MD_coro_outside_frame))
+  if (AI->hasMetadata(LLVMContext::MD_coro_outside_frame))
     return;
 
   // The code that uses lifetime.start intrinsic does not work for functions