ZGC: Remove unnecessary page reset when splitting pages

Split returns a new page (N) of the initial (lower) part and modifies the existing page (P) to be the smaller. Currently, P is reset after splitting by calling ZPage::reset with ZPageResetType::Splitting.

Before split:
[ page ]

After split:
[ return (N) | same page (P) ]

The call hierarchies to ZPage::split_with_pmem are:

P is inserted into the page cache and will be reset with ZPageResetType::Allocation if used later on
ZPageCache::alloc_page
ZPage::split(size_t split_of_size)
ZPage::split(ZPageType type, size_t split_of_size)
ZPage::split_with_pmem

P is in a linked-list to harvest its physical memory when flushing the page cache
ZPageCache::flush
ZPage::split(ZPageType type, size_t split_of_size)
ZPage::split_with_pmem

P is destroyed immediately after calling split
ZPageAllocator::alloc_page_finalize
ZPage::split_committed
ZPage::split_with_pmem

None of these use-cases requires the page (P) to be reset after splitting and thus the reset should be removed.