examples: Drop build-std = ["core"]

Previously the cargo configurations of all of the example projects had
`build-std = ["core"]`, which forces compilation of `core` as a
code-size optimisation. However, this is strictly unnecessary and will
currently break for users who do not use `rustup` directly (e.g. nix
users).

examples/rp/.cargo/config.toml

@@ -1,6 +1,3 @@
-[unstable]
-build-std = ["core"]
-
 [target.'cfg(all(target_arch = "arm", target_os = "none"))']
 runner = "probe-run-rp --chip RP2040"