Disable checkcolor hook

This hook is currently broken (in that it does not fail when a
hard-coded colour is added), and adds 30–40 seconds to the time taken
by repo upload.

Bug: 377902600
Change-Id: I868df74da7e3b6b15e18c9572877dc368ae6f643
Test: Treehugger
Flag: EXEMPT change to pre-upload checks