wasm: remove -wasm-abi= flag

This flag controls whether to convert external i64 parameters for use in
a browser-like environment.

This flag was needed in the past because back then we only supported
wasm on browsers but no WASI. Now, I can't think of a reason why anybody
would want to change the default. For `-target=wasm` (used for
browser-like environments), the wasm_exec.js file expects this
i64-via-stack ABI. For WASI, there is no limitation on i64 values and
`-wasm-abi=generic` is the default.

builder/build.go

@@ -1077,7 +1077,6 @@ func optimizeProgram(mod llvm.Module, config *compileopts.Config) error {
 	// cannot be represented exactly in JavaScript (JS only has doubles). To
 	// keep functions interoperable, pass int64 types as pointers to
 	// stack-allocated values.
-	// Use -wasm-abi=generic to disable this behaviour.
 	if config.WasmAbi() == "js" {
 		err := transform.ExternalInt64AsPtr(mod, config)
 		if err != nil {