pgsql: Remove the pgstats logic for delaying destruction of stats table

Log Message:
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Remove the pgstats logic for delaying destruction of stats table entries.
Per recent discussion, this seems to be making the stats less accurate
rather than more so, particularly on Windows where PID values may be
reused very quickly.  Patch by Peter Brant.

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        autovacuum.c (r1.15 -> r1.16)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c.diff?r1=1.15&r2=1.16)
        pgstat.c (r1.121 -> r1.122)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.121&r2=1.122)
    pgsql/src/include:
        pgstat.h (r1.42 -> r1.43)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h.diff?r1=1.42&r2=1.43)