Remove some dead code from the abi code (#4653)

These were originally used by the old backend framework as part of
legalizing function signatures for the respective ABI.

cranelift/docs/ir.md

@@ -404,7 +404,7 @@ paramlist    : param { "," param }
 retlist      : paramlist
 param        : type [paramext] [paramspecial]
 paramext     : "uext" | "sext"
-paramspecial : "sret" | "link" | "fp" | "csr" | "vmctx" | "sigid" | "stack_limit"
+paramspecial : "sarg" ( num ) | "sret" | "vmctx" | "sigid" | "stack_limit"
 callconv     : "fast" | "cold" | "system_v" | "windows_fastcall"
              | "wasmtime_system_v" | "wasmtime_fastcall"
              | "apple_aarch64" | "wasmtime_apple_aarch64"