RFR: 8343430: RISC-V: C2: Remove old trampoline call

[Fei Yang]

Hi, please review this cleanup change.

The old trampoline call on linux-riscv64 is disabled by default after: https://bugs.openjdk.org/browse/JDK-8332689.
And the new solution (load target address from stub and call) has been baked in jdk master for quite some time.
This propose to remove the related logic of the old trampoline call so that the code will be cleaner and easier to maintain.
As we are still using `trampoline_stub_Relocation` for the stub, I kept using the original `emit_trampoline_stub` name.

Testing on linux-riscv64:
- [x] tier1-tier3 (release)
- [x] hotspot:tier1 (fastdebug)

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Commit messages:
 - Improve
 - Code comment
 - Merge remote-tracking branch 'upstream/master' into JDK-8343430
 - 8343430: RISC-V: Remove old trampoline call

Changes: https://git.openjdk.org/jdk/pull/21822/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21822&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343430
  Stats: 591 lines in 10 files changed: 7 ins; 535 del; 49 mod
  Patch: https://git.openjdk.org/jdk/pull/21822.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21822/head:pull/21822

PR: https://git.openjdk.org/jdk/pull/21822