Remove useless struct that was accidentally added.

(Leftover from development.  Originally, instead of `struct
llama_buffer`, I tried to use `std::vector<UninitU8>`.  This has the
right behavior when compiled normally with full optimizations, but when
building unoptimized, it does a very slow loop where it calls a
constructor for every byte.  Okay, but does performance of unoptimized
builds really matter?  ...Don't know, but in my testing, the same thing
happened when building with optimizations on but with ASan/UBSan
enabled, which is a very useful mode to compile in.  So I figured it was
better to spend a few extra lines of code on `llama_buffer`.)

Author: comex

llama.cpp

@@ -102,10 +102,6 @@ struct llama_layer {
     struct ggml_tensor * w3;
 };
 
-struct UninitU8 {
-    uint8_t c;
-};
-
 struct llama_kv_cache {
     struct ggml_tensor * k;
     struct ggml_tensor * v;