build: remove unused copy regression test script

The copy regression test script is unused, bash-specific and did
not even work properly when it was originally committed.

tests/copycooker.sh

-#!/bin/sh
-
-LC_ALL=C
-export LC_ALL
-
-datadir="tests/data"
-
-logfile="$datadir/copy.regression"
-reffile="$1"
-
-list=$(grep -oh ' ./tests/data/.*' tests/ref/{acodec,lavf,vsynth1}/*| sort)
-rm -f $logfile
-for i in $list ; do
-    echo ---------------- >> $logfile
-    echo $i >> $logfile
-    ./ffmpeg -flags +bitexact -i $i -acodec copy -vcodec copy -y first.nut
-    ./ffmpeg -flags +bitexact -i first.nut -acodec copy -vcodec copy -y second.nut
-    cmp first.nut second.nut >> $logfile
-    md5sum first.nut >> $logfile
-done
-
-if diff -u -w "$reffile" "$logfile" ; then
-    echo
-    echo copy regression test: success
-    exit 0
-else
-    echo
-    echo copy regression test: error
-    exit 1
-fi