pw_varint: remove legacy kMaxVarintSizeBytes
pw_varint: remove legacy kMaxVarintSizeBytes

Removes pw::varint::kMaxVarintSizeBytes now that users have been
moved to kMaxVarint64SizeBytes.

Change-Id: Ibb49faee3adea48c14d1f09aa5bac028559b9f42
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/18741
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Commit-Queue: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>

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