Editor Performance drops when removing frames from animations with curve data

Editor Performance drops when removing frames from animations with curve keys

Regression: No, removing frames on animations with curves had a chance to crash the editor in 4.11, which doesn't happen anymore.

Found while investigating https://udn.unrealengine.com/questions/295322/crash-when-removing-keys-from-the-from-and-end-of.html

1. Create a Third Person Template project
2. Open the ThirdPersonIdle animation and make sure it's playing on loop
3. Right click on about frame 3 in the timeline
4. Select "Remove frame 0 to X"

Result: Frame rate drops to about 15fps
Expected: No performance loss