std: Remove the wasm_syscall feature

This commit removes the `wasm_syscall` feature from the
wasm32-unknown-unknown build of the standard library. This feature was
originally intended to allow an opt-in way to interact with the
operating system in a posix-like way but it was never stabilized.
Nowadays with the advent of the `wasm32-wasi` target that should
entirely replace the intentions of the `wasm_syscall` feature.

src/bootstrap/lib.rs

@@ -498,9 +498,6 @@ impl Build {
         if self.config.profiler {
             features.push_str(" profiler");
         }
-        if self.config.wasm_syscall {
-            features.push_str(" wasm_syscall");
-        }
         features
     }