progress: make display_progress() return void

progress: make display_progress() return void

Ever since the progress infrastructure was introduced in 96a02f8f6d
(common progress display support, 2007-04-18), display_progress() has
returned an int, telling callers whether it updated the progress bar
or not.  However, this is:

  - useless, because over the last dozen years there has never been a
    single caller that cared about that return value.

  - not quite true, because it doesn't print a progress bar when
    running in the background, yet it returns 1; see 85cb8906f0
    (progress: no progress in background, 2015-04-13).

The related display_throughput() function returned void already upon
its introduction in cf84d51c43 (add throughput to progress display,
2007-10-30).

Let's make display_progress() return void, too.  While doing so
several return statements in display() become unnecessary, remove
them.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>