Remove illegal NAOT cctor preinitialization

[MichalPetryka]

Readonly instance fields are not hard immutable, they can be modified with init accessors, in raw IL or reflection.

cc @MichalStrehovsky @jkotas

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[hez2010]

I don't see any wrong with this policy. The "readonly" here doesn't mean the ordinary instance readonly field. Instead, it stands for

• static readonly fields (see StaticReadOnlyFieldPolicy)
• fields that have no actual write operations after IL scanning, including reflection access (see ScannedReadOnlyPolicy)

So _readOnlyPolicy.IsReadOnly(field) should already only be true when the field is really readonly (i.e. no write access to it in the whole program). This is different from ordinary instance readonly field.

[jkotas]

I do not think there is an issue as @hez2010 pointed out. If you believe that there is an issue, please share a repro that demonstrates it.