RFR: 8254781: Remove unimplemented ClassFieldMap::compute_field_count

[Aleksey Shipilev]

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`

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Commit messages:
 - 8254781: Remove unimplemented ClassFieldMap::compute_field_count

Changes: https://git.openjdk.java.net/jdk/pull/663/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=663&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254781
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/663.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/663/head:pull/663

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