Remove stable feature from allowed features list

This CL removes the `default_alloc_error_handler` feature from the
allowed feature set passed to rustc.  This feature was stabilized in
Rust 1.68.0 and now that that has been deployed we can remove the
feature annotations from our codebase.

Bug: 267698452
Test: m rust
Change-Id: Ifc543ec83d3221cc8bcb6c83658abe09696b51ea