pw_log: Remove optional from protos
pw_log: Remove optional from protos

In proto3 all fields are optional by default, with the presence or not
being conveyed by the field having its default, zero, or empty value.

The 'optional' keyword is only required when presence with a default
value is meaningfully different from not present.

Change-Id: I8182ea505a3c06d9718c2526dcb9fda7aee21db0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/97741
Pigweed-Auto-Submit: Scott James Remnant <6b88cbe64f92c5d64ffbf57d7b3d2efc0467fe34@google.com>
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed