Makefile.inc1: garbage collect unused variable: install-info

The variable's definition was removed with texinfo in
2d2813618c3818d7d41a7ced1fca4a1a01d3591d, but the reference to the
variable was left behind.

This is effectively a no-op cleanup.

MFC after: 3 days

(cherry picked from commit de3e5dbea171f576e9e6d89ea5238ea3c9c8b26a)