contrib: remove some scripts in "stats" directory

The "stats" directory contains a couple of scripts to do some statistics
on a repository:

  - "git-common-hash" shows the longest common hash prefixes and can be
    used to determine the minimum prefix length to use for object names
    to be unique. The script has last been touched in 53474eb
    (contrib: update stats/mailmap script, 2012-12-12) and searching for
    it on the internet doesn't really surface any potential use cases or
    even mentions of it.

    Modern Git also shouldn't really need this tool as it knows to
    automatically scale printed prefixes via some heuristics.

  - "mailmap.pl" performs some statistics on the number of mailmapped
    commits in a repository. It has last been modified in 53474eb
    (contrib: update stats/mailmap script, 2012-12-12) and has since
    been bitrotting. It doesn't even compile nowadays anymore:

        $ perl contrib/stats/mailmap.pl
        Experimental keys on scalar is now forbidden at contrib/stats/mailmap.pl line 57.
        Type of arg 1 to keys must be hash or array (not hash element) at contrib/stats/mailmap.pl line 57, near "}) "
        Experimental keys on scalar is now forbidden at contrib/stats/mailmap.pl line 57.
        Type of arg 1 to keys must be hash or array (not private variable) at contrib/stats/mailmap.pl line 57, near "$h)"
        Experimental keys on scalar is now forbidden at contrib/stats/mailmap.pl line 64.
        Type of arg 1 to keys must be hash or array (not private variable) at contrib/stats/mailmap.pl line 64, near "$h)"
        Execution of contrib/stats/mailmap.pl aborted due to compilation errors.

    This should be good-enough signal to indicate that nobody is using
    this script at all anymore.

  - "packinfo.pl" takes the output from git-verify-pack(1) and performs
    some pretty printing thereof. On the one hand it reformats the
    output to be easier to read and provide some summaries. On the other
    hand it may also print filenames of blobs.

    We don't have any replacement for this tool. Ideally, we should move
    its functionality into git-verify-pack(1) itself.

Remove the first two scripts, but retain "packinfo.pl".

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t