Remove redundant capability

We only add an optimization, it must still work with older nodes.

Author: alex-spies

x-pack/plugin/esql/qa/testFixtures/src/main/resources/lookup-join.csv-spec

@@ -146,9 +146,7 @@ emp_no:integer | language_code:integer | language_name:keyword
 
 multipleLookupsAndProjects
 required_capability: join_lookup_v12
-required_capability: join_lookup_enable_pushdown_past_project
 # TODO: this test, but with a lookup index that has multiple matches per row to show the first LOOKUP's effect, too.
-# TODO: tests like this that will look up the exact subfield of a text field
 
 FROM employees
 | KEEP languages, emp_no

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlCapabilities.java

@@ -814,11 +814,6 @@ public enum Cap {
          */
         JOIN_LOOKUP_FIX_LIMIT_PUSHDOWN(JOIN_LOOKUP_V12.isEnabled()),
 
-        /**
-         * Enable Projects (KEEP/DROP and aliasing EVALs) to be pulled up past a LOOKUP JOIN.
-         */
-        JOIN_LOOKUP_ENABLE_PUSHDOWN_PAST_PROJECT(JOIN_LOOKUP_V12.isEnabled()),
-
         /**
          * Fix for https://github.com/elastic/elasticsearch/issues/117054
          */