We were installing every readable file in man/man3, but
only unstalling files matching opus_*.3. Some doxygen
versions construct manpage versions of the marked up
header files and directory index, which got left on the
system after 'make uninstall'.

I don't think the 'uninstall' target is widely used, it's
often broken and can easily break other software. However,
we rely on 'make distcheck' which does verify this issue.

Instead, only install the opus_*.3 manpages, which are the
core api documentation.