Removes the deprecated std::span polyfill, which was only available in
GN and disabled by default.
In a future change, span_common.inc will be merged into pw_span/span.h.
Bug: b/235237667
Change-Id: I0652a295e76c90877d6fbbafa2eae35dcecfacf6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/108732
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Armando Montanez <amontanez@google.com>