Cleanup swapSourceProvider(...) workaround

This reverts the workaround that was introduced in elastic#117792 to avoid EOF error when an es|ql query uses multiple runtime fields that fallback to source when source mode is synthetic.
This is now covered by the `ReinitializingSourceProvider` workaround that covers that and the concurrency problem.
With this change, the main code for the required workarounds are now in isolated in ReinitializingSourceProvider.

server/src/main/java/org/elasticsearch/search/lookup/SearchLookup.java

@@ -102,14 +102,6 @@ private SearchLookup(SearchLookup searchLookup, Set<String> fieldChain) {
         this.fieldLookupProvider = searchLookup.fieldLookupProvider;
     }
 
-    private SearchLookup(SearchLookup searchLookup, SourceProvider sourceProvider, Set<String> fieldChain) {
-        this.fieldChain = Collections.unmodifiableSet(fieldChain);
-        this.sourceProvider = sourceProvider;
-        this.fieldTypeLookup = searchLookup.fieldTypeLookup;
-        this.fieldDataLookup = searchLookup.fieldDataLookup;
-        this.fieldLookupProvider = searchLookup.fieldLookupProvider;
-    }
-
     /**
      * Creates a copy of the current {@link SearchLookup} that looks fields up in the same way, but also tracks field references
      * in order to detect cycles and prevent resolving fields that depend on more than {@link #MAX_FIELD_CHAIN_DEPTH} other fields.
@@ -153,7 +145,4 @@ public Source getSource(LeafReaderContext ctx, int doc) throws IOException {
         return sourceProvider.getSource(ctx, doc);
     }
 
-    public SearchLookup swapSourceProvider(SourceProvider sourceProvider) {
-        return new SearchLookup(this, sourceProvider, fieldChain);
-    }
 }

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/planner/EsPhysicalOperationProviders.java

@@ -39,7 +39,6 @@
 import org.elasticsearch.index.mapper.FieldNamesFieldMapper;
 import org.elasticsearch.index.mapper.MappedFieldType;
 import org.elasticsearch.index.mapper.NestedLookup;
-import org.elasticsearch.index.mapper.SourceFieldMapper;
 import org.elasticsearch.index.mapper.SourceLoader;
 import org.elasticsearch.index.query.QueryBuilder;
 import org.elasticsearch.index.query.QueryBuilders;
@@ -345,16 +344,7 @@ public MappedFieldType.FieldExtractPreference fieldExtractPreference() {
 
                 @Override
                 public SearchLookup lookup() {
-                    boolean syntheticSource = SourceFieldMapper.isSynthetic(indexSettings());
-                    var searchLookup = ctx.lookup();
-                    if (syntheticSource) {
-                        // in the context of scripts and when synthetic source is used the search lookup can't always be reused between
-                        // users of SearchLookup. This is only an issue when scripts fallback to _source, but since we can't always
-                        // accurately determine whether a script uses _source, we should do this for all script usages.
-                        // This lookup() method is only invoked for scripts / runtime fields, so it is ok to do here.
-                        searchLookup = searchLookup.swapSourceProvider(ctx.createSourceProvider());
-                    }
-                    return searchLookup;
+                    return ctx.lookup();
                 }
 
                 @Override