IR: Use remove_if for Instruction::dropUnknownMetadata()

Technically the operations are different -- the old logic moved items
from the back into the opened-up slots, instead of the usual
`remove_if()` logic of a slow and a fast iterator -- but unless a
profile tells us otherwise I prefer the simpler logic here.  Regardless,
there shouldn't be an observable function change.

llvm-svn: 235767