MINOR: buffer: remove the check for output on b_del()

MINOR: buffer: remove the check for output on b_del()

b_del() is used in :
  - mux_h2 with the demux buffer : always processes input data
  - checks with output data though output is not considered at all there
  - b_eat() which is not used anywhere
  - co_skip() where the len is always <= output

Thus the distinction for output data is not needed anymore and the
decrement can be made inconditionally in co_skip().