patch 8.2.0818: Vim9: using a discovery phase doesn't work well

Problem:    Vim9: using a discovery phase doesn't work well.
Solution:   Remove the discovery phase, instead compile a function only when
            it is used.  Add :defcompile to compile def functions earlier.
diff --git a/src/proto/vim9compile.pro b/src/proto/vim9compile.pro
index 6a3d9b5..67c7224 100644
--- a/src/proto/vim9compile.pro
+++ b/src/proto/vim9compile.pro
@@ -9,8 +9,7 @@
 char_u *to_name_const_end(char_u *arg);
 int assignment_len(char_u *p, int *heredoc);
 int check_vim9_unlet(char_u *name);
-int add_def_function(ufunc_T *ufunc);
-void compile_def_function(ufunc_T *ufunc, int set_return_type, cctx_T *outer_cctx);
+int compile_def_function(ufunc_T *ufunc, int set_return_type, cctx_T *outer_cctx);
 void delete_instr(isn_T *isn);
 void delete_def_function(ufunc_T *ufunc);
 void free_def_functions(void);