drop build-dep on libmate-desktop, it's not needed anymore

configure.ac

@@ -47,7 +47,6 @@ AM_GLIB_GNU_GETTEXT
 GLIB_REQUIRED=2.36.0
 GIO_REQUIRED=2.25.12
 DCONF_REQUIRED=0.13.4
-MATE_DESKTOP_REQUIRED=1.9.0
 
 AC_MSG_CHECKING([which gtk+ version to compile against])
 AC_ARG_WITH([gtk],
@@ -81,7 +80,6 @@ PKG_CHECK_MODULES([TERM],
    gio-2.0 >= $GIO_REQUIRED
    gtk+-$GTK_API_VERSION >= $GTK_REQUIRED
    dconf >= $DCONF_REQUIRED
-   mate-desktop-2.0 >= $MATE_DESKTOP_REQUIRED
    x11])
 
 # ********