contrib/wix/defines.wxi

convert: remove unused/unnecessary variable "parentctx"

Differential Revision: https://phab.mercurial-scm.org/D3160

<Include>

  <?if $(var.Platform) = "x64" ?>
    <?define IsX64 = yes ?>
  <?else?>
    <?define IsX64 = no ?>
  <?endif?>

</Include>