Remove redundant PHP_SUBST(EXTRA_LDFLAGS)

This is already done by PHP for shared and static builds.

Author: jmikola

config.m4

@@ -330,7 +330,6 @@ if test "$PHP_MONGODB" != "no"; then
   PHP_NEW_EXTENSION(mongodb, $PHP_MONGODB_SOURCES, $ext_shared,, $PHP_MONGODB_CFLAGS)
 
   PHP_SUBST(MONGODB_SHARED_LIBADD)
-  PHP_SUBST(EXTRA_LDFLAGS)
 
   PHP_ADD_EXTENSION_DEP(mongodb, date)
   PHP_ADD_EXTENSION_DEP(mongodb, json)