Revert "Add rexml dependency

[sichanyoo]

This reverts commit 20ee609.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

1. To make sure we include your contribution in the release notes, please make sure to add description entry for your changes in the "unreleased changes" section of the CHANGELOG.md file (at corresponding gem). For the description entry, please make sure it lives in one line and starts with Feature or Issue in the correct format.

2. For generated code changes, please checkout below instructions first:
   https://github.com/aws/aws-sdk-ruby/blob/version-3/CONTRIBUTING.md

Thank you for your contribution!

[github-actions]

Detected 3 possible performance regressions:

• aws-sdk-core.gem_size_kb - z-score regression: 417.5 -> 418.0. Z-score: Infinity
• aws-sdk-cloudwatchlogs.client_mem_retained - z-score regression: 0.11 -> 0.11. Z-score: Infinity
• aws-sdk-cloudwatchlogs.client_mem_allocated - z-score regression: 0.62 -> 0.62. Z-score: Infinity

[patbl]

Why was this reverted?

[jterapin]

@patbl Hi! There was an internal compatibility issue we needed to unblock before releasing. We're planning to re-commit the change early next week for release.