OvmfPkg: Refer to Shell app via its declared GUID

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>