RFR: 8356924: RISC-V: Clean up cost for vector instructions

[Dingli Zhang]

As mentioned in https://bugs.openjdk.org/browse/JDK-8285790 regarding the ARM64 vector instruct modifications:
Since the new rules are unique and setting different "ins_cost" makes no sense, we have switched to using the default cost.

Currently, there is a similar situation on RISC-V. Over half of the instructions in riscv_v.ad do not include ins_cost definitions. Additionally, as RVV nodes are also unique, we can unify the format by removing these ins_cost entries from riscv_v.ad.

### Testing
qemu-system 9.1.0 with UseRVV (ubuntu24.10):
* [x]  Run test/jdk/jdk/incubator/vector (fastdebug)
* [x]  Run test/hotspot/jtreg/compiler/vectorapi (fastdebug)

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Commit messages:
 - 8356924: RISC-V: Clean up cost for vector instructions

Changes: https://git.openjdk.org/jdk/pull/25221/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25221&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356924
  Stats: 164 lines in 1 file changed: 0 ins; 164 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25221.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25221/head:pull/25221

PR: https://git.openjdk.org/jdk/pull/25221