x86/setup: Remove unused RESERVE_BRK_ARRAY()

Since a13f2ef1 ("x86/xen: remove 32-bit Xen PV guest support"),
RESERVE_BRK_ARRAY() has no user anymore so drop it.

Update related comments too.

Signed-off-by: Cao jin <jojing64@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210311083919.27530-1-jojing64@gmail.com