checker: cleanup err removal (#25043)

vlib/v/checker/fn.v

@@ -3812,15 +3812,15 @@ fn (mut c Checker) check_for_mut_receiver(mut expr ast.Expr) (string, token.Pos)
 }
 
 fn scope_register_it(mut s ast.Scope, pos token.Pos, typ ast.Type) {
-	scope_register_var_name(mut s, pos, typ, 'it')
+	scope_register_special_var_name(mut s, pos, typ, 'it')
 }
 
 fn scope_register_a_b(mut s ast.Scope, pos token.Pos, typ ast.Type) {
-	scope_register_var_name(mut s, pos, typ.ref(), 'a')
-	scope_register_var_name(mut s, pos, typ.ref(), 'b')
+	scope_register_special_var_name(mut s, pos, typ.ref(), 'a')
+	scope_register_special_var_name(mut s, pos, typ.ref(), 'b')
 }
 
-fn scope_register_var_name(mut s ast.Scope, pos token.Pos, typ ast.Type, name string) {
+fn scope_register_special_var_name(mut s ast.Scope, pos token.Pos, typ ast.Type, name string) {
 	s.register(ast.Var{
 		name:       name
 		pos:        pos

vlib/v/gen/c/cgen.v

@@ -7177,7 +7177,7 @@ fn (mut g Gen) or_block(var_name string, or_block ast.OrExpr, return_type ast.Ty
 	}
 	if or_block.kind == .block {
 		g.or_expr_return_type = return_type.clear_option_and_result()
-		if !g.pref.skip_unused || or_block.err_used
+		if or_block.err_used
 			|| (g.fn_decl != unsafe { nil } && (g.fn_decl.is_main || g.fn_decl.is_test)) {
 			g.writeln('\tIError err = ${cvar_name}${tmp_op}err;')
 		}

vlib/v/gen/c/if.v

@@ -311,7 +311,7 @@ fn (mut g Gen) if_expr(node ast.IfExpr) {
 			// define `err` for the last branch after a `if val := opt {...}' guard
 			if is_guard && guard_idx == i - 1 {
 				if err_var := branch.scope.find_var('err') {
-					if !g.pref.skip_unused || err_var.is_used {
+					if err_var.is_used {
 						cvar_name := guard_vars[guard_idx]
 						g.writeln('\tIError err = ${cvar_name}.err;')
 					}