Remove duplicate PTHREADS template define

The undef PTHREADS converts to define if thread safety is configured.
This step is already done by pthreads m4 macros from TSRM so this now
removes duplicated PTHREADS defines from php_config.h.

Author: petk

configure.ac

@@ -105,8 +105,6 @@ extern "C++" {
 #endif
 #endif /* ifndef ZEND_ACCONFIG_H_NO_C_PROTOS */
 
-#undef PTHREADS
-
 #endif /* PHP_CONFIG_H */
 ])