buffer_cache: Remove unused small_vector in CommitAsyncFlushesHigh()

Given this is non-trivial, the constructor is required to execute, so
this removes a bit of redundant codegen.

src/video_core/buffer_cache/buffer_cache.h

@@ -817,7 +817,6 @@ void BufferCache<P>::CommitAsyncFlushesHigh() {
             const std::size_t size = interval.upper() - interval.lower();
             const VAddr cpu_addr = interval.lower();
             ForEachBufferInRange(cpu_addr, size, [&](BufferId buffer_id, Buffer& buffer) {
-                boost::container::small_vector<BufferCopy, 1> copies;
                 buffer.ForEachDownloadRangeAndClear(
                     cpu_addr, size, [&](u64 range_offset, u64 range_size) {
                         const VAddr buffer_addr = buffer.CpuAddr();