Remove the insta-stable cfg(wasm)

The addition of `cfg(wasm)` was an oversight on my end that has a number
of downsides:

* It was introduced as an insta-stable addition, forgoing the usual
  staging mechanism we use for potentially far-reaching changes;
* It is a breaking change for people who are using `--cfg wasm` either
  directly or via cargo for other purposes;
* It is not entirely clear if a bare `wasm` cfg is a right option or
  whether `wasm` family of targets are special enough to warrant
  special-casing these targets specifically.

As for the last point, there appears to be a fair amount of support for
reducing the boilerplate in specifying architectures from the same
family, while ignoring their pointer width. The suggested way forward
would be to propose such a change as a separate RFC as it is potentially
a quite contentious addition.

compiler/rustc_span/src/symbol.rs

@@ -1295,7 +1295,6 @@ symbols! {
         vreg,
         vreg_low16,
         warn,
-        wasm,
         wasm_import_module,
         wasm_target_feature,
         while_let,