Remove StepMake as a separate package

Remove StepMake as a separate package

Rationale:
* there are no publicly-known current usage cases of StepMake outside
LilyPond,
* StepMake installation has been disabled for a decade,
* generic templates (stepmake/stepmake) have been so heavily
customized lately for our heavy documentation that they can no longer
be qualifed as "generic",
* only a few build scripts have remained in stepmake/bin, whereas
most of them now live in scripts/build.

This commit removes everything in stepmake/ except included makefiles;
scripts that are still used are moved to config/ (Autoconf helpers) or
scripts/build (build and install scripts).

Remove config make target to avoid potential conflict with new
config/ directory.

For clarity, a merge of all stepmake templates, from stepmake/stepmake
and make/ into stepmake/ (or similar), will be done in another commit.