RFR: 8318383: Remove duplicated checks in os::get_native_stack() in posix implemenation

[Zhengyu Gu]

Trivial cleanup that removes duplicated checkings.

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Commit messages:
 - Remove duplicated sender_pc check
 - 8318383: Remove duplicated os::is_first_C_frame() check in os::get_native_stack()

Changes: https://git.openjdk.org/jdk/pull/16232/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16232&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318383
  Stats: 6 lines in 1 file changed: 1 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16232.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16232/head:pull/16232

PR: https://git.openjdk.org/jdk/pull/16232