Drop outdated performance.txt file. Fixes #1411 (#1412)

docs/index.txt

@@ -15,7 +15,6 @@ Documentation
 .. toctree::
     :maxdepth: 2
 
-    performance
     protocol
 
     tutorial/index

docs/performance.txt

@@ -1,11 +0,0 @@
-.. _performance:
-
-==============================
-Possible areas for improvement
-==============================
-
-Places for improvement, ordered by difficulty / effectiveness:
-
-* read_zlib() should have a Rust equivalent (~ 4% overhead atm)
-* unpack_object() should have a C equivalent
-