Remove slab dependency

This is used in path.rs, which is dead code until
https://github.com/cloudflare/quiche/commit/3e719a3 which is not yet
included in any released version.

Change-Id: Ic80420770f954ac17e760375481e63b2700c0275

quiche/BUILD.gn

diff --git a/quiche/BUILD.gn b/quiche/BUILD.gn
index d0971bc..8fea0fc 100644
--- a/quiche/BUILD.gn
+++ b/quiche/BUILD.gn
@@ -23,7 +23,6 @@
     "//third_party/rust_crates:libm",
     "//third_party/rust_crates:log",
     "//third_party/rust_crates:ring",
-    "//third_party/rust_crates:slab",
     "//third_party/rust_crates/mirrors/quiche/octets",
   ]
   non_rust_deps = [ "//third_party/boringssl" ]