RFR: 8339248: RISC-V: Remove li64 macro assembler routine and related code

[Fei Yang]

The macro assembler routine li64 and related code (is_li64_at, patch_imm_in_li64, get_target_of_li64 and check_li64_data_dependency) is unused for now. We should remove these unused code, which will save us some unnecessary runtime checks. We can add them back when needed again someday.

Testing:
- [x] release & fastdebug build
- [x] Gtest & Tier1 test (release)

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Commit messages:
 - 8339248: RISC-V: Remove li64 macro assembler routine and related code

Changes: https://git.openjdk.org/jdk/pull/20769/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20769&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339248
  Stats: 111 lines in 2 files changed: 0 ins; 111 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20769.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20769/head:pull/20769

PR: https://git.openjdk.org/jdk/pull/20769