Shell: Fix -Wunreachable-code warnings from clang

Userland/Shell/Builtin.cpp

@@ -416,7 +416,6 @@ int Shell::builtin_exit(int argc, const char** argv)
         printf("Good-bye!\n");
     }
     exit(exit_code);
-    return 0;
 }
 
 int Shell::builtin_export(int argc, const char** argv)

Userland/Shell/Parser.cpp

@@ -161,7 +161,6 @@ NonnullRefPtrVector<AST::Node> Parser::parse_as_multiple_expressions()
             return nodes;
         nodes.append(node.release_nonnull());
     }
-    return nodes;
 }
 
 RefPtr<AST::Node> Parser::parse_toplevel()