patch 8.1.2167: mapping test fails on MS-Windows

Problem:    Mapping test fails on MS-Windows.
Solution:   Remove all the existing Insert-mode mappings.
diff --git a/src/testdir/test_mapping.vim b/src/testdir/test_mapping.vim
index d3ae615..4ba3645 100644
--- a/src/testdir/test_mapping.vim
+++ b/src/testdir/test_mapping.vim
@@ -444,8 +444,8 @@
 endfunc
 
 func Test_list_mappings()
-  " Remove default Mac mapping
-  silent! iunmap <D-v>
+  " Remove default mappings
+  imapclear
 
   inoremap <C-M> CtrlM
   inoremap <A-S> AltS