The HTTPUploadPlugin calls that if encryption is enabled

Call the `encrypt_file` method of the encryption plugin instead

A patch in Gajim made that unnecessary

- unused import
- unused var GAJIM_VERSION
- redundant self.plugin = self
- simplify condition
- redundant 'else'

This happend during porting the plugin to the new encryption API

httpupload version 0.6.2

See merge request !39

ESessions Plugin version 1.0.0

See merge request !36

PGP Plugin version 1.0.0

See merge request !37

omemo version 2.3.0

See merge request !38

Else it could happen that we add the oob tag to a message
that gets later encrypted

This will only be added on not encrypted Messages

Show an error message when OMEMO is activated,
but python-cryptography is missing

This happend due to copy/paste from httpupload

OMEMO Version 2.1.0

See merge request !32