patch 8.1.1901: the +insert_expand feature is not always available

Problem:    The +insert_expand feature is not always available.
Solution:   Graduate the +insert_expand feature.

src/ex_getln.c

@@ -4040,11 +4040,9 @@ open_cmdwin(void)
     /* Don't execute autocommands while creating the window. */
     block_autocmds();
 
-#if defined(FEAT_INS_EXPAND)
     // When using completion in Insert mode with <C-R>=<C-F> one can open the
     // command line window, but we don't want the popup menu then.
     pum_undisplay();
-#endif
 
     /* don't use a new tab page */
     cmdmod.tab = 0;