Remove next to empty backend/CDB README

Found this README while reading some code and rather than fixing
the incorrect path specification it seemed equally useful to just
remove it since it carries very little information.

src/backend/cdb/README

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-CDB2 Additions to PostgreSQL
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-OVERVIEW
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-Directory cdb-pg/src/backend/cdb/ contains CDB2 additions to PostgreSQL.  Corresponding external declarations are in cdb-pg/src/include/cdb/.  
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