Remove NilpotentQuotient method

[olexandr-konovalov]

In gap-system/gap#2513 @fingolfin suggested to remove the following method from LPRES:

InstallOtherMethod( NilpotentQuotient,
  "for an FpGroup using the LPRES-package", true,
  [ IsFpGroup, IsPosInt ], -1, # give priority to NQ package
  function( G, c )
  return( NilpotentQuotient( Range( IsomorphismLpGroup( G ) ), c ) );
  end);

Please see that issue for further details.

[olexandr-konovalov]

Closed by #10.