Remove APIs marked for removal

[offa]

Marked for removal in v0.8.0:

Methods

• [x] InfluxDB::flushBuffer() — #249
• [x] Point::toLineProtocol() — #228
• [x] Point::getCurrentTimestamp() — #228
• [x] InfluxDBException::InfluxDBException(const std::string&, const std::string&) — #249

Types

• [ ] NonExistentDatabase
• [ ] BadRequest
• [ ] ServerError
• [ ] ConnectionError

Disabling deprecation warnings for unit tests might become unnecessary.