RFR: 8274425: Remove unused Modules.extraLimitMods

[Andrey Turbanov]

HashSet `com.sun.tools.javac.comp.Modules.extraLimitMods` is always empty. Code only check its content, but no content added.
There was a method `addExtraLimitModules`, but it was removed under [JDK-8175191](https://bugs.openjdk.java.net/browse/JDK-8175191) 253fdcd0b26
Method `addExtraLimitModules` itself was added in ec9ca2997f4, but was never actually used .

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Commit messages:
 - [PATCH] Remove unused Modules.extraLimitMods

Changes: https://git.openjdk.java.net/jdk/pull/5628/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5628&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274425
  Stats: 11 lines in 1 file changed: 0 ins; 7 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5628.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5628/head:pull/5628

PR: https://git.openjdk.java.net/jdk/pull/5628