pgsql: Get rid of adjust_appendrel_attr_needed(), which has been broken

Log Message:
-----------
Get rid of adjust_appendrel_attr_needed(), which has been broken ever since
we extended the appendrel mechanism to support UNION ALL optimization. The
reason nobody noticed was that we are not actually using attr_needed data for
appendrel children; hence it seems more reasonable to rip it out than fix it.
Back-patch to 8.2 because an Assert failure is possible in corner cases.
Per examination of an example from Jim Nasby.

In HEAD, also get rid of AppendRelInfo.col_mappings, which is quite inadequate
to represent UNION ALL situations; depend entirely on translated_vars instead.

Modified Files:
--------------
pgsql/src/backend/nodes:
copyfuncs.c (r1.410 -> r1.411)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c?r1=1.410&r2=1.411)
equalfuncs.c (r1.335 -> r1.336)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c?r1=1.335&r2=1.336)
outfuncs.c (r1.343 -> r1.344)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/outfuncs.c?r1=1.343&r2=1.344)
pgsql/src/backend/optimizer/path:
allpaths.c (r1.175 -> r1.176)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/allpaths.c?r1=1.175&r2=1.176)
pgsql/src/backend/optimizer/prep:
prepjointree.c (r1.58 -> r1.59)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepjointree.c?r1=1.58&r2=1.59)
prepunion.c (r1.160 -> r1.161)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepunion.c?r1=1.160&r2=1.161)
pgsql/src/include/nodes:
relation.h (r1.163 -> r1.164)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/relation.h?r1=1.163&r2=1.164)
pgsql/src/include/optimizer:
prep.h (r1.62 -> r1.63)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/prep.h?r1=1.62&r2=1.63)