patch 8.2.1612: Vim9: cannot pass "true" to prop_remove()

Problem:    Vim9: cannot pass "true" to prop_remove().
Solution:   Use dict_get_bool(). (closes #6853)
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index 012b3eb..f771812 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -321,6 +321,15 @@
   bwipe!
 endfunc
 
+def Test_prop_remove_vim9()
+  new
+  call AddPropTypes()
+  call SetupPropsInFirstLine()
+  call assert_equal(1, prop_remove({'type': 'three', 'id': 13, 'both': true, 'all': true}))
+  call DeletePropTypes()
+  bwipe!
+enddef
+
 func SetupOneLine()
   call setline(1, 'xonex xtwoxx')
   normal gg0