patch 8.2.1882: Vim9: v:disallow_let is no longer needed

Problem:    Vim9: v:disallow_let is no longer needed.
Solution:   Remove v:disallow_let.
diff --git a/src/evalvars.c b/src/evalvars.c
index fd67ad0..b3ad2c9 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -146,7 +146,6 @@
     {VV_NAME("echospace",	 VAR_NUMBER), VV_RO},
     {VV_NAME("argv",		 VAR_LIST), VV_RO},
     {VV_NAME("collate",		 VAR_STRING), VV_RO},
-    {VV_NAME("disallow_let",	 VAR_NUMBER), 0}, // TODO: remove
 };
 
 // shorthand
@@ -243,9 +242,6 @@
 
     set_vim_var_nr(VV_ECHOSPACE,    sc_col - 1);
 
-    // TODO: remove later
-    set_vim_var_nr(VV_DISALLOW_LET, 1);
-
     // Default for v:register is not 0 but '"'.  This is adjusted once the
     // clipboard has been setup by calling reset_reg_var().
     set_reg_var(0);
@@ -749,8 +745,7 @@
 	    ex_finally(eap);
 	    return;
     }
-    if (get_vim_var_nr(VV_DISALLOW_LET)
-				      && eap->cmdidx == CMD_let && vim9script)
+    if (eap->cmdidx == CMD_let && vim9script)
     {
 	emsg(_(e_cannot_use_let_in_vim9_script));
 	return;