FINERACT-2801: Remove unused (dead) code in fineract-document

[SamaSVM]

Description

Removes unused code from fineract-document only.

Removed code with no references found anywhere in the codebase, including dedicated unit tests:

• Unused classes and tests: FileContentDetector and FileContentDetectorTest
• Unused exception: ContentPathSanitizerException
• Unused content processors and tests: GzipDecoderContentProcessor, GzipEncoderContentProcessor, SizeContentProcessor, and ContentProcessorTest

No functional changes were introduced; application behavior remains unaffected.

Checklist

Please make sure these boxes are checked before submitting your pull request - thanks!

• Write the commit message as per our guidelines
• Acknowledge that we will not review PRs that are not passing the build ("green") - it is your responsibility to get a proposed PR to pass the build, not primarily the project's maintainers.
• Create/update unit or integration tests for verifying the changes made.
• Follow our coding conventions.
• Add required Swagger annotation and update API documentation at fineract-provider/src/main/resources/static/legacy-docs/apiLive.htm with details of any API changes
• This PR must not be a "code dump". Large changes can be made in a branch, with assistance. Ask for help on the developer mailing list.
• If merging this PR resolves a JIRA issue, I will mark that issue as resolved and set "Fix Version/s" appropriately.

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