I didn't see the underscore so
Code:
AMD_TabOnCharacterFrame_Options.MiddleActive:Hide()
Should work.
Depending on where you put this in your code, you might need to make sure your .xml file is listed above your .lua file in the .toc (the files get loaded in order and the frame in the .xml file is created when the file is loaded).
It's not going to look the way you want though as hiding the MiddleActive texture carves a notch in the top of the tab.
You either need to live with the gold line or make a texture to replace the one being used for MiddleActive or create your own tab button and textures.