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
------
REL8_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/67b5effe8cf693590a0cc71b457b750ab34fa6ec

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

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