efi_loader: do not call efi_runtime_detach twice

Commit 7f95104d ("efi_loader: detach runtime in ExitBootServices()")
added a call to efi_runtime_detach() to ExitBootServices() but did not
remove the call in SetVirtualAddressMap().

Remove the superfluous function call.

Correct a comment referring to efi_runtime_detach().

Fixes: 7f95104d ("efi_loader: detach runtime in ExitBootServices()")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>