[wasm] Stop requiring TZDIR and TZDEFAULT on WASI

[kateinoigakukun]

Those constants are not used on WASI in Swift side, so there's no need to require them in the C headers. The #error was introduced by 03fe46f.

[kateinoigakukun]

@swift-ci test

[parkera]

#elseif os(WASI)
            // WASI doesn't provide a way to get the current timezone for now, so
            // just return the default GMT timezone.

I guess time zones just don't exist in this environment?

[kateinoigakukun]

Yes, the platform doesn't have the time zone concept.