Remove ExternalStoreDB::getSlave

It has been deprecated since 1.34 and it is unused.

Change-Id: I4771e31baadf38b96ef54efe36e30958ea56e1d4

RELEASE-NOTES-1.39

@@ -259,6 +259,8 @@ because of Phabricator reports.
   service container.
 * ApiQueryBase::showHiddenUsersAddBlockInfo(), deprecated in 1.34, has been
   removed. Use ApiQueryBlockInfoTrait instead.
+* ExternalStoreDB::getSlave(), deprecated in 1.34, has been removed. Use
+  ExternalStoreDB::getReplica() instead.
 * …
 
 === Deprecations in 1.39 ===

includes/externalstore/ExternalStoreDB.php

@@ -171,18 +171,6 @@ class ExternalStoreDB extends ExternalStoreMedium {
 		);
 	}
 
-	/**
-	 * Get a replica DB connection for the specified cluster
-	 *
-	 * @param string $cluster Cluster name
-	 * @return DBConnRef
-	 * @deprecated since 1.34
-	 */
-	public function getSlave( $cluster ) {
-		wfDeprecated( __METHOD__, '1.34' );
-		return $this->getReplica( $cluster );
-	}
-
 	/**
 	 * Get a primary database connection for the specified cluster
 	 *