Remove the geometry callback functions

Removing this because it's not properly supported for dynamic
extension loading. The 2 SpatiaLite RTree functions require you to
compile SQLite into the SpatiaLite library which isn't possible with
the current way this module works.

More info [here](http://trac.osgeo.org/osgeo4w/ticket/305)

    SQLite introduced the sqlite3_rtree_geometry_callback() API about
    one or two years ago; so if you are using some very old libsqlite3
    this symbol will not be defined.

    Please note: SpatiaLite discourages using the Geometry Callback
    based interface, and will be disabled by default in any new
    version.
    so the suggested solution when building splite is to always define:

    DISABLE_GEOMETRY_CALLBACKS

    bye Sandro

This reverts commit 589f558710cf323d9c73e57e3f7f1912c9f69395.