Eliminate the parser_use_ast switch, which does nothing, and

exec_no_exec, which also does nothing in the new parser

parser.cpp

@@ -1205,16 +1205,3 @@ scope_block_t::scope_block_t(block_type_t type) : block_t(type)
 breakpoint_block_t::breakpoint_block_t() : block_t(BREAKPOINT)
 {
 }
-
-bool parser_use_ast(void)
-{
-    env_var_t var = env_get_string(L"fish_new_parser");
-    if (var.missing_or_empty())
-    {
-        return 1;
-    }
-    else
-    {
-        return from_string<bool>(var);
-    }
-}