zlib: remove mentions of unexposed Z_TREES constant

`Z_TREES` is never exposed on `DefineZlibConstants()`, hence it shall be
removed.

This fix is applicable to v18+.

Refs: https://github.com/nodejs/node/blob/main/src/node_zlib.cc

Author: hkleungai

doc/api/zlib.md

@@ -615,7 +615,6 @@ Allowed flush values.
 * `zlib.constants.Z_FULL_FLUSH`
 * `zlib.constants.Z_FINISH`
 * `zlib.constants.Z_BLOCK`
-* `zlib.constants.Z_TREES`
 
 Return codes for the compression/decompression functions. Negative
 values are errors, positive values are used for special but normal

lib/zlib.js

@@ -304,7 +304,7 @@ ZlibBase.prototype._final = function(callback) {
 // This is currently only used to figure out which flush flag to use for the
 // last chunk.
 // Roughly, the following holds:
-// Z_NO_FLUSH (< Z_TREES) < Z_BLOCK < Z_PARTIAL_FLUSH <
+// Z_NO_FLUSH < Z_BLOCK < Z_PARTIAL_FLUSH <
 //     Z_SYNC_FLUSH < Z_FULL_FLUSH < Z_FINISH
 const flushiness = [];
 const kFlushFlagList = [Z_NO_FLUSH, Z_BLOCK, Z_PARTIAL_FLUSH,