Remove memory management comparison (#1049)

This is a follow-up to #998 and the discussion in #1049. The
comparison page is now gone: like @randomPoison said, it feels
redundant and I also mostly skip over it when teaching the class.

I also took out some duplication in the Rust memory management page. I
would be up for simplifying the whole chapter down to one or two
slides as @djmitche suggests: that would leave us with more time for
covering ownership.

src/SUMMARY.md

@@ -85,7 +85,6 @@
   - [Scope-Based Memory Management](memory-management/scope-based.md)
   - [Garbage Collection](memory-management/garbage-collection.md)
   - [Rust Memory Management](memory-management/rust.md)
-  - [Comparison](memory-management/comparison.md)
 - [Ownership](ownership.md)
   - [Move Semantics](ownership/move-semantics.md)
   - [Moved Strings in Rust](ownership/moved-strings-rust.md)

src/memory-management/rust.md

@@ -3,7 +3,6 @@
 Memory management in Rust is a mix:
 
 * Safe and correct like Java, but without a garbage collector.
-* Depending on which abstraction (or combination of abstractions) you choose, can be a single unique pointer, reference counted, or atomically reference counted.
 * Scope-based like C++, but the compiler enforces full adherence.
 * A Rust user can choose the right abstraction for the situation, some even have no cost at runtime like C.