Remove outdated comment

Comment referred to the headers installation step when the
PHP_INSTALL_HEADERS wasn't yet available:
9e9f413

Author: petk

ext/pdo/config.m4

@@ -12,7 +12,5 @@ if test "$PHP_PDO" != "no"; then
   PHP_NEW_EXTENSION(pdo, pdo.c pdo_dbh.c pdo_stmt.c pdo_sql_parser.c pdo_sqlstate.c, $ext_shared)
   PHP_ADD_EXTENSION_DEP(pdo, spl, true)
   PHP_INSTALL_HEADERS([ext/pdo], [php_pdo.h php_pdo_driver.h php_pdo_error.h])
-
-  dnl so we always include the known-good working hack.
   PHP_ADD_MAKEFILE_FRAGMENT
 fi