OvmfPkg: Refer to Shell app via its declared GUID

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1843

Currently, the file GUID reference of the UEFI Shell app is indirected
via the PCD gEfiIntelFrameworkModulePkgTokenSpaceGuid.PcdShellFile,
which is set to a fixed value for OvmfPkg.

So instead, use the symbolic GUID in ShellPkg for this purpose, and drop
the reference to this PCD, and to the IntelFrameworkModulePkg package
entirely.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

OvmfPkg/OvmfPkgX64.dsc

@@ -508,8 +508,6 @@
   #
 !include NetworkPkg/NetworkPcds.dsc.inc
 
-  gEfiIntelFrameworkModulePkgTokenSpaceGuid.PcdShellFile|{ 0x83, 0xA5, 0x04, 0x7C, 0x3E, 0x9E, 0x1C, 0x4F, 0xAD, 0x65, 0xE0, 0x52, 0x68, 0xD0, 0xB4, 0xD1 }
-
 !if $(SMM_REQUIRE) == TRUE
   gUefiCpuPkgTokenSpaceGuid.PcdCpuSmmStackSize|0x4000
 !endif