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().