Remove redundant PHP_SUBST_OLD(INCLUDE_PATH) (#12738)

The AC_SUBST(INCLUDE_PATH) is enough to only replace the variable in the
build-defs.h.in file.

Author: petk

configure.ac

@@ -1555,7 +1555,6 @@ PHP_SUBST_OLD(EXTRA_LIBS)
 PHP_SUBST_OLD(ZEND_EXTRA_LIBS)
 PHP_SUBST_OLD(INCLUDES)
 PHP_SUBST_OLD(EXTRA_INCLUDES)
-PHP_SUBST_OLD(INCLUDE_PATH)
 PHP_SUBST_OLD(INSTALL_IT)
 PHP_SUBST(LFLAGS)
 PHP_SUBST(LIBTOOL)