Remove host function definition region

[syntactically]

The host function definition region is not often used, and we hope it will be used less going forward as we move toward more use of IDL/deciding interfaces at compilation time. The use in hyperlight-wasm module mode can be replaced by a host function call, which does not require statically deciding on a dedicated memory region size.

[jsturtevant]

This was done in #453 and brought back in #600. Could be a useful reference