mm: drop SECTION_SHIFT in code comments

Actually SECTIONS_SHIFT is used in the kernel code, so the code comments
is strictly incorrect.  And since commit bbeae5b0

 ("mm: move page
flags layout to separate header"), SECTIONS_SHIFT definition has been
moved to include/linux/page-flags-layout.h, since code itself looks quite
straighforward, instead of moving the code comment into the new place as
well, we just simply remove it.

This also fixed a checkpatch complain derived from the original code:
WARNING: please, no space before tabs
+ * SECTIONS_SHIFT    ^I^I#bits space required to store a section #$

Link: https://lkml.kernel.org/r/20210531091908.1738465-2-aisheng.dong@nxp.com
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Suggested-by: Yu Zhao <yuzhao@google.com>
Reviewed-by: Yu Zhao <yuzhao@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>