patch 8.2.0913: code for resetting v:register is duplicated

Problem:    Code for resetting v:register is duplicated.
Solution:   Add reset_reg_var().
diff --git a/src/main.c b/src/main.c
index 201ed37..d66cd0c 100644
--- a/src/main.c
+++ b/src/main.c
@@ -826,12 +826,7 @@
     // done after the clipboard is available and all initial commands that may
     // modify the 'clipboard' setting have run; i.e. just before entering the
     // main loop.
-    {
-	int default_regname = 0;
-
-	adjust_clip_reg(&default_regname);
-	set_reg_var(default_regname);
-    }
+    reset_reg_var();
 #endif
 
 #if defined(FEAT_DIFF)