x86: mmu: don't decrement z_free_page_count in reserving code

In z_mem_manage_init(), z_free_page_count is only manipulated
after all reserved pages are marked, and will reflect
the actual number of page frames being added to the free page
frame list. Manipulating z_free_page_count before this is
going to mess up the accounting, so remove the code to
decrement z_free_page_count in arch_reserved_pages_update()
under x86.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>