Remove text about enums from ReferenceEquality check, as it doesn't c…

…heck them.

I considered rephrasing it to say that using #equals is preferred but not enforced by this check, but mentioning them at all seems confusing.

Fixes #783

RELNOTES: Tweak documentation for ReferenceEquality

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=212956380

docs/bugpattern/ReferenceEquality.md

@@ -15,12 +15,6 @@ Well, no, because some tricky client can always generate a new instance with
 `new Boolean(true)`. Comparing with `equals` always works; comparing with `==`
 doesn't.
 
-### But `enum` values are always unique, so can't I compare _them_ with `==`?
-
-Yes, _but_ that might confuse the reader, who must understand that your type has
-special properties because it's an `enum`. Using `equals` everywhere can work
-the same everywhere; special-casing for enums isn't worth it.
-
 ### How about a reference equality comparison before a more expensive content equality comparison?
 
 The check allows implementations of `Object#equals()` to perform reference