Remove non-Core error constants

[newpavlov]

Some error constants are used only in libc and can not emerge from Core functions. See this comment.

Continuation of bytecodealliance/wasi-rs#7

[linclark]

Thanks for filing this. Once WASI modularization is complete, we'll be refactoring the errors so that each module only contains the errors that are relevant to its functions. Through that process, we will be cleaning up any unused errors, so I'm going to close this one out.