install_plpgsql is no longer a regress test (it's done via createlang);

src/test/regress/expected/Makefile

src/test/regress/input/Makefile

src/test/regress/input/install_plpgsql.source

src/test/regress/output/Makefile

src/test/regress/output/install_plpgsql.source

src/test/regress/sql/Makefile