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.

compileopts/options.go

@@ -42,7 +42,6 @@ type Options struct {
 	PrintAllocs     *regexp.Regexp // regexp string
 	PrintStacks     bool
 	Tags            []string
-	WasmAbi         string
 	GlobalValues    map[string]map[string]string // map[pkgpath]map[varname]value
 	TestConfig      TestConfig
 	Programmer      string