I created an autoload script named MCP. I later said to myself, "Self. That was stupid. It should be a parent class." So, I deleted MCP from the autoload list, saved the contents of the .gd file, and deleted the .gd file. I then tried to create a Node2D named MCP with a script of mcp.gd, and the script had a "class_name MCP" in it. The editor did not like that in any way, shape or form. Wound up naming it Mcp.

Nothing named MCP exists in my project, but if I enter mc, then the editor continues to show MCP as an autocompletion. It's like it never left.
Does the editor have a cache of some sort I can clear?

Yeah, it's the .godot hidden folder in the project. Sometimes it gets outta sync. You can delete it and it will autoregenerate. Save your work, close the editor, and make a backup of the folder first just in case.