Hide "Library Content" and "Problem Bank" buttons in Legacy Library UI.

[bradenmacdonald]

Per @kdmccormick: "New library blocks and ProblemBank blocks can both be added to Legacy Content Libraries with big green buttons. Even if that managed to work correctly, we shouldn't be encouraging users to do this."

[navinkarkera]

@jmakowski1123 @lizc577 @sdaitzman @marcotuts This is ready for AC testing on the sandbox

[sdaitzman]

No longer appearing for me, looks good!