Makefile.inc1: remove ncurses/form from _prebuild_libs

As of d287d3282f43 libbsddialog has a built-in form implementation and
does not need a formw dependency.

This reverts commit 483a226238ed8949c6d280ae0757a0683962a74b.

Reported by: asiciliano
Sponsored by: The FreeBSD Foundation