Remove alleged support for last-millennium Unixes

Remove workarounds for NeXTStep (last released in 1995), SunOS (1994),
HP-UX 9.x (1992) and 10.x (1995), OSF/1 / Digital UNIX / Tru64 UNIX
4.x (1999), and AIX 4.x (1999).

HP-UX 11 implements dlopen(), so dropping support for earlier versions
also lets us remove the HP-UX-specific gmodule-dld.

https://bugzilla.gnome.org/show_bug.cgi?id=710519