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

arch/x86/include/asm/pgtable_64.h

@@ -175,16 +175,13 @@ extern void sync_global_pgds(unsigned long start, unsigned long end);
  * and a page entry and page directory to the page they refer to.
  */
 
-/*
- * Level 4 access.
- */
-#define mk_kernel_pgd(address) __pgd((address) | _KERNPG_TABLE)
+/* PGD - Level 4 access */
 
-/* PUD - Level3 access */
+/* PUD - Level 3 access */
 
-/* PMD  - Level 2 access */
+/* PMD - Level 2 access */
 
-/* PTE - Level 1 access. */
+/* PTE - Level 1 access */
 
 /*
  * Encode and de-code a swap entry