RFR: 8254575: C2: Clean up unused TRACK_PHI_INPUTS assertion code

[Roberto Castañeda Lozano]

Remove assertion code that was disabled in all build configurations.

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Commit messages:
 - 8254575: C2: Clean up unused TRACK_PHI_INPUTS assertion code

Changes: https://git.openjdk.java.net/jdk/pull/606/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=606&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254575
  Stats: 23 lines in 1 file changed: 0 ins; 23 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/606.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/606/head:pull/606

PR: https://git.openjdk.java.net/jdk/pull/606