pgsql: Remove ancient text file containing plpgsql installation

Log Message:
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Remove ancient text file containing plpgsql installation instructions.
This was long ago superseded by the standard build process and main
SGML documentation.

Removed Files:
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pgsql/src/pl/plpgsql/src:
INSTALL
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/INSTALL)