Deprecate and obsolete OldSize

The purpose served by `OldSize` can be achieved using `InitialHeapSize` and `NewSize` (or `NewRatio`) due to the intrinsic constraint: `NewSize + OldSize == InitialHeapSize`. When users specify all three flags with improper values, thereby violating this constraint, one flag value must be discarded. Discarding any one of them might come as a surprise to users. The presence of `OldSize` as a command-line flag introduces a fair amount of complexity, evident in `GenArguments::initialize_size_info`.

This flag is exclusively utilized by the Serial/Parallel GC.