MdeModulePkg/Core: fix too many available pages between BS_Data

The root cause is an unnecessary check to Size parameter in function
AdjustMemoryS(). It will cause one standalone free page (happen to have
Guard page around) in the free memory list cannot be allocated, even if
the requested memory size is less than a page.

  //
  // At least one more page needed for Guard page.
  //
  if (Size < (SizeRequested + EFI_PAGES_TO_SIZE (1))) {
    return 0;
  }

The following code in the same function actually covers above check
implicitly. So the fix is simply removing above check.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>