Drop speculations about advLayout and schema.

The content is still roughly accurate, but... well, speculative.

The other parts of this old doc fragment (package layout details) are
now in the newer more complete schema doc file, so, can be dropped.
(Which would leave *only* speculations in this file, so...)
Signed-off-by: Eric Myhre <hash@exultant.us>

doc/dev.md

-Developer Docs
-==============
-
-### Schema system packages
-
-// Note: this is somewhat more aspirational as yet.
-
-- `go-ipld-prime/typed` -- functions for operating over the typed nodes.
-- `go-ipld-prime/typed/ast` -- a (self-describing!) tree of schema declarations.
-	- n.b. `ast.Type` nodes have to be reified into `typed.Type` to be used; this involves running a completeness check on the whole set of `ast.Node` at once (e.g. making sure no references dangle, etc).
-- `go-ipld-prime/typed/ast/parser` -- (future work) a parser for the schema DSL.
-- `go-ipld-prime/typed/ast/parser/fmt` -- (future work) a canonicalizing printer for the schema DSL.
-- `go-ipld-prime/advlay/chamt` -- (future work) "advanced layouts" for CHAMT.
-
-Note the relationship between the `advlay` and the `typed` families of packages
-is not fully fleshed out.  It seems likely that the Schema system will provide
-a good clear place to indicate and configure the use of Advanced Layouts, but
-perhaps we'll additionally find a good API that allows Advanced Layouts to be
-exposed as an `ipld.Node` that works with purely Data Model layer concepts too.
-(All Advanced Layouts we've considered so far present as either a map, array,
-or byte range; so making them work as pure Data Model should be feasible.)