[lworld] RFR: 8377328: [lworld] Remove legacy readme and classfile constants

[Chen Liang]

There is a legacy src/readme and legacy ACC_PRIMITIVE and Q descriptor left in classfile_constants.h.template. Let's remove all of them.

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Commit messages:
 - Remove obsolete constants and readme

Changes: https://git.openjdk.org/valhalla/pull/2057/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2057&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377328
  Stats: 53 lines in 2 files changed: 0 ins; 53 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/2057.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2057/head:pull/2057

PR: https://git.openjdk.org/valhalla/pull/2057