lib: move call to _n_m_invalidate_metadata

lib: move call to _n_m_invalidate_metadata

It is wrong most of the time in _notmuch_message_remove_terms, but
that function is too low level to know how to call
_n_m_invalidate_metadata with the right argument, at least not without
more extensive changes. This change merely makes the current behaviour
more obvious, since the other calls cannot have relied on metadata
being invalidated.