Remove no-longer-used system_symbols

This is a follow-up to the cleanup done in 2725a05929. Creating a
system_symbol_libraryt every time a languaget is created (effectively everytime
from_expr is called) is costly, because of all the strings being looked up in
the string table. On SV-COMP benchmarks, this accounted for 2% of runtime.
Furthermore this removes this undesirable dependency on goto-programs from
langapi.

src/langapi/language.h

@@ -12,18 +12,17 @@ Author: Daniel Kroening, kroening@kroening.com
 #ifndef CPROVER_LANGAPI_LANGUAGE_H
 #define CPROVER_LANGAPI_LANGUAGE_H
 
-#include <unordered_set>
 #include <iosfwd>
-#include <string>
 #include <memory> // unique_ptr
+#include <set>
+#include <string>
+#include <unordered_set>
 
 #include <util/message.h>
 #include <util/std_types.h>
 #include <util/symbol.h>
 #include <util/symbol_table_base.h>
 
-#include <goto-programs/system_library_symbols.h>
-
 class symbol_tablet;
 class exprt;
 class namespacet;
@@ -188,7 +187,6 @@ public:
 
 protected:
   bool language_options_initialized=false;
-  system_library_symbolst system_symbols;
 };
 
 #endif // CPROVER_UTIL_LANGUAGE_H

src/langapi/module_dependencies.txt

@@ -1,3 +1,2 @@
-goto-programs
 langapi
 util