doc: delete non-existing zlib constants

Some constants in the zlib docs are not in the actual code:

zlib.Z_BINARY
zlib.Z_TEXT
zlib.Z_ASCII
zlib.Z_UNKNOWN

Also handled in https://github.com/nodejs/node/pull/7203, but marked as
semver-major, so will not land in v6.x.

Fixes: https://github.com/nodejs/node/issues/7204
PR-URL: https://github.com/nodejs/node/pull/7520
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

doc/api/zlib.md

@@ -239,13 +239,6 @@ Compression strategy.
 * `zlib.Z_FIXED`
 * `zlib.Z_DEFAULT_STRATEGY`
 
-Possible values of the data_type field.
-
-* `zlib.Z_BINARY`
-* `zlib.Z_TEXT`
-* `zlib.Z_ASCII`
-* `zlib.Z_UNKNOWN`
-
 The deflate compression method (the only one supported in this version).
 
 * `zlib.Z_DEFLATED`