Clean up install-uzbl-browser makefile rule

First of all the `@PREFIX@` in the uzbl-browser.in and uzbl.desktop.in files is already repleaced by the makefile rule `bin/uzbl-browser` and `uzbl.desktop` so there is no need to do that twice. In addition to that the mode of the examples scripts doesn't need to be modified after they have been installed to `$(INSTALLDIR)/share/uzbl/examples/data/scripts/*` since 755 is already the mode of the relevant examples scripts in the repo.