pgsql: Remove inaccurate "Incrementally Updated Backups" documentation

Remove inaccurate "Incrementally Updated Backups" documentation section;
already removed from 9.0+.

Applied to 8.3 and 8.4.

Branch
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REL8_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/8b812177752f01fa2a1fdf59e438e82d832750e1

Modified Files
--------------
doc/src/sgml/backup.sgml | 40 ----------------------------------------
1 files changed, 0 insertions(+), 40 deletions(-)

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