Remove unused <extension>_SHARED_LIBADD variables (#13361)

These are used to store libraries needed to link when extension is
shared and needs libraries linked to it. Since other similar types of
extensions don't have these variables substituted in Makefile, these can
be also removed. The filter and mysqli don't need any libraries for
linking here.

ext/filter/config.m4

@@ -6,7 +6,6 @@ PHP_ARG_ENABLE([filter],
 
 if test "$PHP_FILTER" != "no"; then
   PHP_NEW_EXTENSION(filter, filter.c sanitizing_filters.c logical_filters.c callback_filter.c, $ext_shared,, -DZEND_ENABLE_STATIC_TSRMLS_CACHE=1)
-  PHP_SUBST(FILTER_SHARED_LIBADD)
 
   PHP_INSTALL_HEADERS([ext/filter], [php_filter.h])
   PHP_ADD_EXTENSION_DEP(filter, pcre)

ext/mysqli/config.m4

@@ -64,7 +64,6 @@ if test "$PHP_MYSQLI" != "no"; then
                   mysqli_report.c mysqli_driver.c mysqli_warning.c \
                   mysqli_exception.c mysqli_result_iterator.c"
   PHP_NEW_EXTENSION(mysqli, $mysqli_sources, $ext_shared,, -DZEND_ENABLE_STATIC_TSRMLS_CACHE=1)
-  PHP_SUBST(MYSQLI_SHARED_LIBADD)
   PHP_INSTALL_HEADERS([ext/mysqli], [php_mysqli_structs.h mysqli_mysqlnd.h])
   PHP_ADD_EXTENSION_DEP(mysqli, mysqlnd)
 fi