Remove conditional attribute switch from rules_rust

Currently Bazel is using native `CcToolchainInfo` - meaning Java implementation of the provider.
There is an ongoing effort to rewrite provider in Starlark. Once that happens and Bazel knows of Starlark version of the provider remove conditional switch so that it always points to Starlark attribute. So instead of:

```
if hasattr(cc_toolchain, "_linker_files"):
linker_depset = cc_toolchain._linker_files
else:
linker_depset = cc_toolchain.linker_files()
```

We would have:
```
linker_depset = cc_toolchain._linker_files
```