[arcmt] Remove the "it is not safe to remove an unused 'autorelease' message" ARC

migration error.

This is more trouble that it is worth; autoreleasing a value without holding on it
is a valid use-case, we should not "punish" correct code for the minority of
broken/fragile programs that depend on the behavior of  -autorelease.

rdar://9914061

llvm-svn: 156999