This function removes the N-th component from L; it changes the value of L.
Use the function WithoutNth to create a new list containing the
elements of L except the N-th (without changing L).
Example
/**/ use R ::= QQ[x,y,z];
/**/ L := indets(R);
/**/ L;
[x, y, z]
/**/ remove(ref L,2);
/**/ L;
[x, z]