RFR: 8258696: Temporarily revert use of pattern match instanceof until docs-reference is fixed

[Chris Hegarty]

Temporarily revert use of pattern match instanceof construct until docs-reference is fixed, see JDK-8258657. 

... 
Generating REFERENCE_API javadoc for 21 modules 

            if (delegate instanceof ExecutorService service) { 
                                                    ^ 
  (use --enable-preview to enable pattern matching in instanceof) 
1 error 
make[3]: *** [/Users/chhegar/git/open/build/macosx-x64/support/docs/_javadoc_REFERENCE_API_exec.marker] Error 1 
Docs.gmk:472: recipe for target '/Users/chhegar/git/open/build/macosx-x64/support/docs/_javadoc_REFERENCE_API_exec.marker' failed 
make/Main.gmk:485: recipe for target 'docs-reference-api-javadoc' failed 
make[2]: *** [docs-reference-api-javadoc] Error 2 
make[2]: *** Waiting for unfinished jobs....

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Commit messages:
 - avoid instanceof pattern match

Changes: https://git.openjdk.java.net/jdk/pull/1843/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1843&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258696
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1843.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1843/head:pull/1843

PR: https://git.openjdk.java.net/jdk/pull/1843