MdeModulePkg/BdsDxe: Don't delete "BootNext" until booting it

MdeModulePkg/BdsDxe: Don't delete "BootNext" until booting it

Current implementation deletes the "BootNext" before calling
any PlatformBootManagerLib APIs, but if system resets in
PlatformBootManagerLib APIs, "BootNext" is not consumed but lost.

The patch defers the deletion of "BootNext" to before booting it.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Sunny Wang <sunnywang@hpe.com>