ShellPkg/UefiShellLib: drop DeviceHandle param of ShellOpenFileByDevicePath()

The ShellOpenFileByDevicePath() API promises to set the DeviceHandle
output parameter to the handle of the filesystem identified by the
FilePath input parameter. However, this doesn't actually happen when the
UEFI Shell 2.0 method is used (which is basically "always" nowadays).

Accordingly, the only caller of ShellOpenFileByDevicePath(), namely
ShellOpenFileByName(), defines a (dummy) local DeviceHandle variable just
so it can call ShellOpenFileByDevicePath().

Remove the useless output parameter.

Cc: Jaben Carsey <jaben.carsey@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1008
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>