x86/mm: Remove the unused mk_kernel_pgd() #define

AFAICS the last uses of directly 'making' kernel PGDs was removed 7 years ago:

  8b78c21d: ("x86, 64bit, mm: hibernate use generic mapping_init")

Where the explicit PGD walking loop was replaced with kernel_ident_mapping_init()
calls. This was then (unnecessarily) carried over through the 5-level paging conversion.

Also clean up the 'level' comments a bit, to convey the original, meanwhile somewhat
bit-rotten notion, that these are empty comment blocks with no methods to handle any
of the levels except the PTE level.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200724114418.629021-4-mingo@kernel.org