fftools/ffmpeg: disable and deprecate -qphist

This option adds a long string of numbers to the progress line, where
i-th number contains the base-2 logarithm of the number of times a frame
with this QP value was seen by print_report().

There are multiple problems with this feature:
* despite this existing since 2005, web search shows no indication
  that it was ever useful for any meaningful purpose;
* the format of what is printed is entirely undocumented, one has to
  find it out from the source code;
* QP values above 31 are silently ignored;
* it only works with one video stream;
* as it relies on global state, it is in conflict with ongoing
  architectural changes.

It then seems that the nontrivial cost of maintaining this option is not
worth its negligible (or possibly negative - since it pollutes the
already large option space) value.
Users who really need similar functionality can also implement it
themselves using -vstats.

doc/ffmpeg.texi

@@ -1051,8 +1051,6 @@ Specify which version of the vstats format to use. Default is @code{2}. See the
 top=1/bottom=0/auto=-1 field first
 @item -vtag @var{fourcc/tag} (@emph{output})
 Force video tag/fourcc. This is an alias for @code{-tag:v}.
-@item -qphist (@emph{global})
-Show QP histogram
 @item -vbsf @var{bitstream_filter}
 Deprecated see -bsf