[Rust Server] Retire mimetypes modules

Created by: richardwhiuk

This removes the mimetypes module, because:

• it doesn't really add any value (it used to when we used lazy_static! to initialise MimeType structs but they're not just &strs)
• it's possible to get collisions in the identifiers.

This fix was contributed by @mirw and reviewed by myself on behalf of @Metaswitch.

Rust Server Technical Committee

• @frol
• @farcaller
• @bjgill

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
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