util: remove compressors dict (API)

util: remove compressors dict (API)

We no longer have any in-tree consumers of this object. Use
util.compengines instead.

--- a/mercurial/util.py	Mon Nov 07 18:35:43 2016 -0800
+++ b/mercurial/util.py	Mon Nov 07 18:34:51 2016 -0800
@@ -3070,15 +3070,6 @@
 
 compengines.register(_noopengine())
 
-compressors = {
-    None: nocompress,
-    # lambda to prevent early import
-    'BZ': lambda: bz2.BZ2Compressor(),
-    'GZ': lambda: zlib.compressobj(),
-    }
-# also support the old form by courtesies
-compressors['UN'] = compressors[None]
-
 def _makedecompressor(decompcls):
     def generator(f):
         d = decompcls()