[perception] Fix link error in thread sanitizer build (RobotLocomotio…

…n#17925)

- Test case: bazel test --config=clang --compilation_mode=dbg --config=tsan \
             //tools/install/libdrake:drake_shared_library_test
- The link error was introduced by absl/container:flat_hash_map.
- Use std::unordered_map instead.

perception/BUILD.bazel

@@ -41,7 +41,6 @@ drake_cc_library(
     ],
     deps = [
         "//common:unused",
-        "@abseil_cpp_internal//absl/container:flat_hash_map",
     ],
 )
 

perception/point_cloud.cc

@@ -2,10 +2,10 @@
 
 #include <algorithm>
 #include <functional>
+#include <unordered_map>
 #include <utility>
 #include <vector>
 
-#include "absl/container/flat_hash_map.h"
 #include <fmt/format.h>
 #include <fmt/ostream.h>
 
@@ -419,7 +419,7 @@ PointCloud PointCloud::VoxelizedDownSample(double voxel_size) const {
   }
 
   // Create a map from voxel coordinate to a set of points.
-  absl::flat_hash_map<Eigen::Vector3i, std::vector<int>, Vector3iHash>
+  std::unordered_map<Eigen::Vector3i, std::vector<int>, Vector3iHash>
       voxel_map;
   for (int i = 0; i < size_; ++i) {
     if (xyz(i).array().isFinite().all()) {