x86/boot: Drop 5-level paging related variables and early updates

The variable __pgtable_l5_enabled is no longer used so it can be
dropped.

Along with it, drop ptrs_per_p4d and pgdir_shift, and replace any
references to those with expressions based on pgtable_l5_enabled(). This
ensures that all observers see values that are mutually consistent.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>