[riscv-port] RFR: 8278531: riscv: Remove duplicate code bitset_to_fregs

[Feilong Jiang]

After JDK-8278337, the definition of `bitset_to_fregs` is just same as `bitset_to_regs`.

Hotspot and jdk tier1 passed on the unmatched board. All tests were tested without new failures.

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Commit messages:
 - 8278531: riscv: Remove duplicate code bitset_to_fregs

Changes: https://git.openjdk.java.net/riscv-port/pull/28/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=28&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278531
  Stats: 17 lines in 2 files changed: 0 ins; 14 del; 3 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/28.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/28/head:pull/28

PR: https://git.openjdk.java.net/riscv-port/pull/28