Remove unused errors from errno enum

[linclark]

Currently, witx provides a single errno enum that contains all of the errors across the different proposals that are currently in the WebAssembly/wasi repo.

Once WASI modularization is complete and these proposals are in different modules, we should go through and determine which errors from the errno enum are actually relevant to the functions contained in each repo. For some modules, such as wasi-clock and wasi-random, there will be very few (possibly zero) applicable errors. Other proposals, such as wasi-filesystem and wasi-poll, will have many more.

Wasi-libc will take on the task of converting the errors provided by the different modules into something usable by POSIX, etc. The work in wasi-libc will happen after the next snapshot.