Remove partial screenshot code

It's broken currently, is extremely difficult to access, and will
need to be heavily rewritten/refactored anyway if we decide to
bring it back.

Fix: 249325548
Test: adb shell input keycombination -t 300 META_LEFT CTRL_LEFT
SHIFT_LEFT S
(now takes a regular screenshot)

Change-Id: I2cde564f6369b88bd4c6b20a9eecf8172fae7e7a