Remove FileTypes enum and all references

[joneubank]

Summary of request

With the implementation of #852 there is no longer a need for the FileTypes enum:
https://github.com/overture-stack/SONG/blob/develop/song-core/src/main/java/bio/overture/song/core/model/enums/FileTypes.java

Details

We want to remove this enum from the code base.

This has a few inconsequential references, in the score-server FileEntity class, and in several Test cases. We can update these references, removing the overloaded function that accepts FileTypes argument, and replacing their use in test case with plain strings.