OvmfPkg/TdxMailboxLib: Delete global variables

OvmfPkg/TdxMailboxLib: Delete global variables

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4172

TdxMailboxLib once was designed to be used in DXE phase. But now it is
going to be used in SEC/PEI phase (in the following patches). Global
variables are not allowed. The library is refactored after those global
variables are deleted.

Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>