Remove obsolete variable

* src/Makefile.am (gpg4win_pkg_pinentry_configure): Remove
$(pinentry_enable_disable).

This variable was added with f5081e7ba1f12ba09fb31ed61d9f7b58c932aa4e to
include pinentry-qt4 in the full installer. It was removed with
5e427a7e7de086d70a2677d9a0dc0f56bbf6161b.