[crypto-rustcrypto]: remove unused custom feature (#161)

The `custom` feature from `getrandom` is not actually used in normal
builds — infact, `getrandom` itself is never used directly.

However, the feature can be used by a binary on an embedded platform
to register a custom random number generator. The final binary should
enable the `custom` feature to do this, see the
`register_custem_genrandom!` macro for more details:

  https://docs.rs/getrandom/latest/getrandom/macro.register_custom_getrandom.html

We emulate this in the CI build by enabling the feature on the command
line.

Co-authored-by: mulmarta <103590845+mulmarta@users.noreply.github.com>

.github/workflows/no_std_build.yml

@@ -26,4 +26,4 @@ jobs:
     - name: Build MLS Embedded Full RFC Compliance
       run: cargo +nightly build --package mls-rs --lib --no-default-features --features rfc_compliant --target thumbv6m-none-eabi
     - name: Build rust crypto embedded
-      run: cargo +nightly build --package mls-rs-crypto-rustcrypto --no-default-features --target thumbv6m-none-eabi
+      run: cargo +nightly build --package mls-rs-crypto-rustcrypto --no-default-features --features "getrandom/custom" --target thumbv6m-none-eabi

mls-rs-crypto-rustcrypto/Cargo.toml

@@ -37,7 +37,7 @@ thiserror = { version = "1.0.40", optional = true }
 zeroize = { version = "1", default-features = false, features = ["alloc", "zeroize_derive"] }
 
 # Random
-getrandom = { version = "0.2", default-features = false, features = ["custom"] }
+getrandom = { version = "0.2", default-features = false, optional = true }
 rand_core = { version = "0.6", default-features = false, features = ["alloc"] }
 
 # AEAD