x86/setup: Clean up the header portion of setup.c

In 20 years we accumulated 89 #include lines in setup.c,
but we only need 30 of them (!) ...

Get rid of the excessive ones, and while at it, sort the
remaining ones alphabetically.

Also get rid of the incomplete changelogs at the header of the file,
and explain better what this file does.

Cc: linux-kernel@vger.kernel.org
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>