This removes both the pl/pgsql version and the shared library version
of the `tile_for_point` SQL function. This function was only used in some
old migrations, and is not required for production usage.
Removing this function simplifies the installation and configuration for
new developers.
These SQL functions are separate from the `tile_for_point` ruby/C function
which is part of the quad_tile gem. This function is still used when
creating and updating database records.
Fixes#3110.
Instead, we define models in the migrations themselves. This ensures that
column names etc match the state of the database during the migration,
not the current live version of the model.
