Eliminate rustc_check_flags in favor of extra_rustc_flags

Summary:
None of the current usages of `rustc_check_flags` was used in such a way that flags only being passed to `check` builds made sense. They were all just using it as a poor substitute for `extra_rustc_flags`, i.e. add flags without erasing all the existing flags configured by the toolchain's `rustc_flags`.

The semantics of `rustc_check_flags` is confusing to begin with. Those flags are not passed just to `…[check]` builds. They are passed also for dependencies during pipelined non-check builds, because those also involve building metadata.

This diff deletes support for `rustc_check_flags` in `RustToolchainInfo` and switches existing uses to `extra_rustc_flags`.

Reviewed By: zertosh, diliop

Differential Revision: D55438348

fbshipit-source-id: 221e1b6944774eebbeebac6f25882c9024b7b579

example/.buckconfig

@@ -16,4 +16,3 @@ target_platform_detector_spec = target:root//...->prelude//platforms:default
 [rust]
 default_edition = 2021
 remap_src_paths = yes
-rustc_check_flags = --cap-lints=warn