Don't try to install mod_tile.so using debian/libapache2-mod-tile.install

This was not working anymore since 273973e which moved the sources to a src/
directory in order to clean up the top level directory, which broke the package
build. However, debian/rules already contains calls to 'make' which install
mod_tile.so to the correct location, so it's not even necessary to install it
using debian/libapache2-mod-tile.install.