Maintainers.txt: Remove unused OvmfPkg Confidential Computing path

The following commit removed PlatformBootManagerLibGub from
OvmfPkg.  Update Maintainers.txt to remove reference to
deleted directory.

tianocore/edk2@6fb2760

Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Michael Roth <michael.roth@amd.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>

Maintainers.txt

@@ -497,7 +497,6 @@ F: OvmfPkg/Include/Guid/ConfidentialComputingSecret.h
 F: OvmfPkg/Include/Library/MemEncryptSevLib.h
 F: OvmfPkg/IoMmuDxe/CcIoMmu.*
 F: OvmfPkg/Library/BaseMemEncryptSevLib/
-F: OvmfPkg/Library/PlatformBootManagerLibGrub/
 F: OvmfPkg/Library/CcExitLib/
 F: OvmfPkg/PlatformPei/AmdSev.c
 F: OvmfPkg/ResetVector/