[analyzer] Remove strip of ElementRegion in CallEvent::invalidateRegions.

This is a bit of old code trying to deal with the fact that functions that
take pointers often use them to access an entire array via pointer
arithmetic. However, RegionStore already conservatively assumes you can use
pointer arithmetic to access any part of a region.

Some day we may want to go back to handling this specifically for calls,
but we can do that in the future.

No functionality change.

llvm-svn: 177569