Remove obsolete code in examples

These lines of code were commented out and can be safely removed.

examples/permutations.lp

@@ -1,5 +1,4 @@
 #show p/2.
-%#external integer(n(0)).
 
 {p(1..n, 1..n)}.
 

examples/prime.lp

@@ -1,5 +1,4 @@
 #show prime/1.
-%#external integer(n(0)).
 
 composite(I * J) :- I = 2..n, J = 2..n.
 prime(N) :- N = 2..n, not composite(N).

examples/schur-numbers.lp

@@ -1,6 +1,4 @@
 #show in/2.
-%#external integer(n(0)).
-%#external integer(r(0)).
 
 {in(1..n, 1..r)}.
 covered(I) :- in(I, S).