contrib: remove git-convert-objects

contrib: remove git-convert-objects

git-convert-objects, originally named git-convert-cache was used in
early 2005 to convert ancient repositories where objects are named
after the hash of their compressed contents to the current object
naming sheme where they are named after the hash of their pre-compression
contents.

By now the need for conversion of the very early repositories is
less relevant, we no longer need to keep it in contrib; remove it.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>