Remove backwards compat ugliness in snapbuild.c.

Remove backwards compat ugliness in snapbuild.c.

In 955a684e040 we fixed a bug in initial snapshot creation. In the
course of which several members of struct SnapBuild were obsoleted. As
SnapBuild is serialized to disk we couldn't change the memory layout.

Unfortunately I subsequently forgot about removing the backward compat
gunk, but luckily Heikki just reminded me.

This commit bumps SNAPBUILD_VERSION, therefore breaking existing
slots (which is fine in a major release).

Author: Andres Freund
Reminded-By: Heikki Linnakangas <[email protected]>
Discussion: https://postgr.es/m/c94be044-818f-15e3-1ad3-7a7ae2dfed0a@iki.fi