BUG/MINOR: mux-quic: remove extra BUG_ON() in _qcc_send_stream()

BUG/MINOR: mux-quic: remove extra BUG_ON() in _qcc_send_stream()

The following patch fixed a BUG_ON() which could be triggered if RS/SS
emission was scheduled after stream local closure.
  7ee1279f4b8416435faba5cb93a9be713f52e4df
  BUG/MEDIUM: mux-quic: fix crash on RS/SS emission if already close local

qcc_send_stream() was rewritten as a wrapper around an internal
_qcc_send_stream() used to bypass the faulty BUG_ON(). However, an extra
unnecessary BUG_ON() was added by mistake in _qcc_send_stream().

This should not cause any issue, as the BUG_ON() is only active if <urg>
argument is false, which is not the case for RS/SS emission. However,
this patch is labelled as a bug as this BUG_ON() is unnecessary and may
cause issues in the future.

This should be backported up to 2.8, after the above mentionned patch.