[Ref] Remove unused _colours property

[braders]

Overview

[Ref] Remove unused _colours property.

Comments

Multiple factors make me think this property is unused.

• Searching the codebase for _colours returns no other options,
• The commit which introduced the array contains the phrase "WIP" (work in progress),
• The property is marked as private, and so I don't think there is a risk of this breaking extensions.

[civibot]

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