remove unused writeUtf8 and readUtf8 from TProtocol.py

[vzhd1701]

cleanup after #3105

writeUtf8 and readUtf8 are not referenced anywhere in the code any more

• Did you create an Apache Jira ticket? (Request account here, not required for trivial changes)
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[fishy]

Can you please rebase on top of latest master branch so we can rerun CI? There was an issue with CI that's fixed in #3124.