Remove no-op handling of non-existent dynamic_size expression

There is no code that creates an expression with id "dynamic_size" and
the three places handling this id just did nothing.

Author: tautschnig

src/analyses/invariant_set.cpp

@@ -1014,9 +1014,6 @@ void invariant_sett::modifies(const exprt &lhs)
   else if(lhs.id()=="valid_object")
   {
   }
-  else if(lhs.id()=="dynamic_size")
-  {
-  }
   else if(lhs.id()==ID_byte_extract_little_endian ||
           lhs.id()==ID_byte_extract_big_endian)
   {

src/pointer-analysis/value_set.cpp

@@ -1391,7 +1391,6 @@ void value_sett::assign_rec(
       add_to_sets);
   }
   else if(lhs.id()=="valid_object" ||
-          lhs.id()=="dynamic_size" ||
           lhs.id()=="dynamic_type" ||
           lhs.id()=="is_zero_string" ||
           lhs.id()=="zero_string" ||

src/pointer-analysis/value_set_fi.cpp

@@ -1219,7 +1219,6 @@ void value_set_fit::assign_rec(
       recursion_set);
   }
   else if(lhs.id()=="valid_object" ||
-          lhs.id()=="dynamic_size" ||
           lhs.id()=="dynamic_type")
   {
     // we ignore this here