Remove ancient text file containing plpgsql installation instructions.

This was long ago superseded by the standard build process and main
SGML documentation.

src/pl/plpgsql/src/INSTALL

-Installation of PL/pgSQL
-
-1)	Type 'make' to build the shared plpgsql object.
-
-2)	Type 'make install' to install the shared object in
-	the PostgreSQL library directory.
-
-3)	Declare the PL/pgSQL procedural language in your
-	database by running the createlang script:
-
-		createlang plpgsql DBNAME
-
-	If you declare the language in the template1 database,
-	any subsequently created database will have PL/pgSQL
-	support installed automatically.