SecurityPkg/SecureBootConfigDxe: Fix deleting signature data issue.

SecurityPkg/SecureBootConfigDxe: Fix deleting signature data issue.

Replace "(UINT8 *)NewVariableData" with (UINT8 *)NewVariableData + Offset"
to avoid the header of EFI_SIGNATURE_LIST being copied to the front of
NewVariableData every time and update ListWalker when handling the current
EFI_SIGNATURE_LIST finishes.

Cc: Zhang Chao <chao.b.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: chenc2 <chen.a.chen@intel.com>
Reviewed-by: Zhang Chao <chao.b.zhang@intel.com>