Drop support for ige-mac-integration

See the discussion at !67 (closed).

modified: configure.ac modified: libbalsa/Makefile.am modified: libbalsa/identity.c modified: libbalsa/libbalsa-gpgme-cb.c modified: libbalsa/libbalsa-gpgme.c deleted: libbalsa/macosx-helpers.c deleted: libbalsa/macosx-helpers.h modified: libbalsa/meson.build modified: libbalsa/rfc3156.c modified: libbalsa/smtp-server.c modified: libbalsa/source-viewer.c modified: meson.build modified: meson_options.txt modified: src/ab-main.c modified: src/address-book-config.c modified: src/balsa-app.c modified: src/balsa-index.c modified: src/balsa-mblist.c modified: src/balsa-message.c modified: src/balsa-mime-widget-callbacks.c modified: src/filter-edit-callbacks.c modified: src/folder-conf.c modified: src/information-dialog.c modified: src/mailbox-conf.c modified: src/main-window.c modified: src/message-window.c modified: src/pref-manager.c modified: src/sendmsg-window.c modified: src/store-address.c modified: src/toolbar-prefs.c