py: Rename __QSTR_EXTRACT flag to NO_QSTR.

It has more usages than just qstr extraction, for example, embedding (where
people don't care about efficient predefined qstrs).
diff --git a/py/qstr.h b/py/qstr.h
index 1989ebe..8c63fbb 100644
--- a/py/qstr.h
+++ b/py/qstr.h
@@ -37,7 +37,7 @@
 
 // first entry in enum will be MP_QSTR_NULL=0, which indicates invalid/no qstr
 enum {
-#ifndef __QSTR_EXTRACT
+#ifndef NO_QSTR
 #define QDEF(id, str) id,
 #include "genhdr/qstrdefs.generated.h"
 #undef QDEF