Remove packed-simd benchmark

Upstream changes in rustc have made this no longer compile, and looking at the
README description (quoted below) and a quick perf profile did not indicate any
particularly interesting features that would make it worth continuing to
feature.

> Portable packed SIMD vectors. Exercises SIMD features heavily, and uses a compiler plugin.

This was added back in #268 but the reasons cited for its inclusion there don't
seem to actually reflect the current state of the benchmark (we never evaluated
test compilation time). I suspect that re-adding a more recent copy of stdsimd
may be relevant, but we indirectly measure compilation time there through the
self-bootstrap testing, so this is mostly duplicate work regardless.

Author: Mark-Simulacrum

collector/benchmarks/README.md

@@ -43,7 +43,7 @@ These are real programs that are known to stress the compiler in interesting
 ways.
 
 - **diesel**: A type save SQL query builder. Utilizes the type system to
-  ensure a lot of invariants. Stresses anything related to resolving 
+  ensure a lot of invariants. Stresses anything related to resolving
   trait bounds, by having a lot of trait impls for a large number of different
   types.
 - **encoding**: Character encoding support. Contains some large tables.
@@ -52,8 +52,6 @@ ways.
   compiler in certain ways.
 - **keccak**: A cryptography algorithm. Contains a very high number of locals
   and basic blocks.
-- **packed-simd**: Portable packed SIMD vectors. Exercises SIMD features
-  heavily, and uses a compiler plugin.
 - **ucd**: A Unicode crate. Contains large statics that
   [stress](https://github.com/rust-lang/rust/issues/53643) the borrow checker's
   implementation of NLL.
@@ -118,4 +116,4 @@ programs.
   behavior](https://github.com/rust-lang/rust/issues/65510) (in the number of
   associated type bounds).
 - **externs**: A large amount of extern functions has caused [slowdowns in the past](https://github.com/rust-lang/rust/pull/78448).
-- **derive**: A large amount of simple structs with a `#[derive]` attribute for common built-in traits such as Copy and Debug. 
+- **derive**: A large amount of simple structs with a `#[derive]` attribute for common built-in traits such as Copy and Debug.