SourceField properties compress and compress_threshold removed in 5.0

[sethmlarson]

These properties are available in 2.0 but not in 5.0 and onwards:

• 2.0: https://github.com/elastic/elasticsearch/blob/2.0/core/src/main/java/org/elasticsearch/index/mapper/internal/SourceFieldMapper.java#L97-L99
• 5.0: https://github.com/elastic/elasticsearch/blob/5.0/core/src/main/java/org/elasticsearch/index/mapper/SourceFieldMapper.java

Thus we should remove them from the SourceField class. Probably were inherited via NEST and due to being optional didn't make any noise when they went away.