Remove `sentry_timestamp` header

[antonpirker]

Problem Statement

Currently the SDK sends the sentry_timestamp header but it is ignored on the relay side, so it just leads to confusion what this is for. So we should remove it from the code

Solution Brainstorm

Remove the header here: https://github.com/getsentry/sentry-python/blob/master/sentry_sdk/utils.py#L278
Also remove the timestamp parameter of the to_header() function because it is not used anymore and can lead to confusion. (and it is not set anywhere anyhow)