[DeadArgElim] Remove allocsize attributes on callsites

We're already removing allocsize attributes from Functions that we
remove args from, since removing arguments from a function may make the
allocsize attribute incorrect. It appears we forgot to also remove them
from callsites.

Without this, I get verifier errors on `@Test2`.

It probably wouldn't be too hard to make DAE properly update allocsize
attributes instead of dropping them, but I can't think of a scenario
where that'd be useful in practice.

llvm-svn: 329868