Remove libreadline from the source tree, all consumers but gdb
has been switched to libedit long ago, libreadline was built as an
internallib for a while and kept only for gdbtui which was broken using
libreadline.

Since gdb has been mostly deorbitted in all arches, gdbtui was only installed
on arm and sparc64, given it has been removed, gdb has been switched to use
libedit, no consumers are left for libreadline. Thus this removal