Glib::thread_init has been deprecated since 2.32,ib
is no longer necessary and no longer has any effect.
This fixes builds with modern glib[mm] versions, notably
with msys2 in Windows.
This is mostly a simple lexical search+replace but the absence of operator< for
std::weak_ptr<T> leads to some complications, particularly with Evoral::Sequence
and ExportPortChannel.
