patch 8.2.0605: Vim9: cannot unlet an environment variable

Problem:    Vim9: cannot unlet an environment variable.
Solution:   Implement unlet for $VAR.
diff --git a/src/version.c b/src/version.c
index 8902126..7af2e50 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    605,
+/**/
     604,
 /**/
     603,