SecurityPkg: Allow VariablePolicy state to delete authenticated variables

SecurityPkg: Allow VariablePolicy state to delete authenticated variables

https://bugzilla.tianocore.org/show_bug.cgi?id=2522

Causes AuthService to check
IsVariablePolicyEnabled() before enforcing
write protections to allow variable deletion
when policy engine is disabled.

Only allows deletion, not modification.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Bret Barkelew <brbarkel@microsoft.com>
Signed-off-by: Bret Barkelew <brbarkel@microsoft.com>
Reviewed-by: Dandan Bi <dandan.bi@intel.com>
Acked-by: Jian J Wang <jian.j.wang@intel.com>