RFR: 8254781: Remove unimplemented ClassFieldMap::compute_field_count

[Paul Hohensee]

On Wed, 14 Oct 2020 15:27:26 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> There is no definition of `ClassFieldMap::compute_field_count` in current tip or any history after the initial load.
> Can be removed.
> Testing:
>  - [x] Linux x86_64 build
>  - [x] Text searches for `compute_field_count` in `src/hotspot`

Marked as reviewed by phh (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/663