remove unused test env var from TESTING doc

TESTING.asciidoc

@@ -33,12 +33,6 @@ Use local transport:
 export ES_TEST_LOCAL=true && mvn test
 -------------------------------------
 
-Wait on mapping changes:
-
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-export ES_WAIT_ON_MAPPING_CHANGE=true && mvn test
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-
 === Test case filtering.
 
 - `tests.class` is a class-filtering shell-like glob pattern,