gdb/NEWS: announce removal of QNX Neutrino support

QNX Neutrino support was removed here [1], but I forgot to mention in in
NEWS.

[1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=36fb20fa93484b104d91e42e38930ee8629192ab



Change-Id: I8db7957acdd0be3c1e0b751c7c245870c4cd7101
Approved-By: Eli Zaretskii <eliz@gnu.org>