Remove check for dnet_addr in dnet_stub library (#12611)

The dnet_stub library was once used on some obsolete systems and was
needed for transitive linking with Sybase and some PHP extensions. At
this point, no known and supported system uses the dnet_stub library
name, so it's safe to remove this check. This also came in pdo_dblib as
a copy/paste from other removed PHP extensions.

Partially was already removed in 1b969a7.

Author: petk

ext/pdo_dblib/config.m4

@@ -45,9 +45,6 @@ if test "$PHP_PDO_DBLIB" != "no"; then
 
   PDO_DBLIB_DEFS="-DPDO_DBLIB_FLAVOUR=\\\"freetds\\\""
   PHP_NEW_EXTENSION(pdo_dblib, pdo_dblib.c dblib_driver.c dblib_stmt.c, $ext_shared,,-I$pdo_cv_inc_path $PDO_DBLIB_DEFS -DZEND_ENABLE_STATIC_TSRMLS_CACHE=1)
-  AC_CHECK_LIB(dnet_stub, dnet_addr, [
-    PHP_ADD_LIBRARY_WITH_PATH(dnet_stub,,PDO_DBLIB_SHARED_LIBADD)
-  ])
   AC_DEFINE(HAVE_PDO_DBLIB,1,[ ])
   PHP_SUBST(PDO_DBLIB_SHARED_LIBADD)