Remove useless comments

The name of the functions already explain what we are doing.

Author: romainbrenguier

src/solvers/prop/prop_conv.cpp

@@ -61,10 +61,7 @@ literalt prop_conv_solvert::get_literal(const irep_idt &identifier)
   if(!result.second)
     return result.first->second;
 
-  // produce new variable
   literalt literal=prop.new_variable();
-
-  // set the name of the new variable
   prop.set_variable_name(literal, id2string(identifier));
 
   // insert