Postgres updater fixes to make update.php able to run

Postgres updater fixes to make update.php able to run

* Remove redundant schema prefix from relname=x query. The
  schema filtering is already done via the JOIN. The relname
  portion is just the table name not <schema>.<table name>.
* Avoid explicit table schema qualification and rely on the
  search path, as MW 1.27 did. Previously it only used the
  global $wgDBschema var to pass to determineCoreSchema()
  instead of keeping it in mSchema.
* Clean up some code duplication in Database::tableName() and
  make the code comments clearer.
* Make DatabasePostgres::tableName() use parent::tableName()
  instead of a method that just wraps this method. The intent
  seems clearer this way.
* Remove unused return value in
  PostgresUpdater::rebuildTextSearch().

Bug: T148628
Change-Id: Id11d9576b7c2fdad22ff7f90727c12997217a632