Remove libde265 as it contains patent-encumbered HEVC-related logic

[kleisauke]

See for discussion libvips/build-win64-mxe#13 and libvips/build-win64-mxe#14.

/cc @lovell

[lovell]

To reduce future maintenance, are we at a point where the MXE-based https://github.com/libvips/build-win64-mxe/ repo might allow us to consider deprecating this repo?

[jcupitt]

I find jhbuild easier to work with than makefiles, but I might be the only one :)

The other reason for keeping jhbuild would be for building win32 versions of nip2 -- as a gnome project, jhbuild comes with useful stuff to build the gtk stack. Perhaps mxe has that too.