Remove All Base64 References from GTFOBins Database

[calebstewart]

Description of Changes

The base64 stream type in gtfobins.json is no longer valid, but there were entries left behind using it. This pull request is meant to remove all remnants of that stream type due to the updates to LinuxFile which now supports raw data without any encoding.

I've tested the changes so far with the Peak Hill room, but I need to go through and test each possible GTFObin transfer method to make sure it's working properly. Before merging this, I want to write proper unit tests that will test each one explicitly to make sure they all work.

Fixes #173.

Major Changes Implemented:

• Remove GTFOBins base64 stream type

Pre-Merge Tasks

• Formatted all modified files w/ python-black
• Sorted imports for modified files w/ isort
• Ran flake8 on repo, and fixed any new problems w/ modified files
• Ran pytest test cases
• Added brief summary of updates to CHANGELOG (under [Unreleased])

For issues with pre-merge tasks, see CONTRIBUTING.md