distrib: Remove mention of no-longer-existent make target

Summary:
`show-install-setup` used to be defined by `distrib/Makefile` which hasn't been
used in a very long time and was removed from the tree in
a1c008b3.

Reviewers: monoidal

Reviewed By: monoidal

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5194

distrib/INSTALL

@@ -22,9 +22,6 @@ Now run:
 
 	make install 
 
-(`make show-install-setup' prints the details of where the different
-pieces of the bundle are heading when -- possibly helpful).
-
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 main GHC site: