patch 8.1.1159: MS-Windows: with a silent (un)install $VIM/_vimrc is removed

Problem:    MS-Windows: with a silent (un)install $VIM/_vimrc is removed.
Solution:   Don't delete _vimrc in silent mode. (Ken Takata, closes #4242)
diff --git a/nsis/gvim.nsi b/nsis/gvim.nsi
index 8336093..2e990b8 100644
--- a/nsis/gvim.nsi
+++ b/nsis/gvim.nsi
@@ -1096,7 +1096,9 @@
 	Call un.GetParent
 	Pop $0
 
-	Delete $0\_vimrc
+	${IfNot} ${Silent}
+	  Delete $0\_vimrc
+	${Endif}
 	RMDir $0
 SectionEnd