Remove the realArgumentsCount member in CallContext

Remove the realArgumentsCount member in CallContext

The data is easily available through the CallData, only used by
the Arguments object and we save a pointer in Heap::CallData this way.

For this to work, let CallData::argc always return the real number
of arguments passed into the function.

Change-Id: I59c7c41e8c1af160db09fa794977ab7084c9e12d
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>