Remove availableExternally logic and extra inlining semantic passes.

Turns out that since the change to use DMD's symbol emission
logic a few releases back we didn't actually ever emit code
for the extra modules. Thus, all the effort for the extra
semantic passes went to waste.

If we want to implement DMD-style cross-module inlining, we
need to add the extra semantic3s back in, and then iterate
over all the non-root modules with a codegen visitor that
only emits symbols as available_externally.

We probably won't need the availableExternally members if
we start with a clean design in the new symbol emission
framework. Thus, remove everything in the meantime. This
will make some release builds a bit faster, and should not
negatively affect generated code.