Remove unused templated instances (rapidsai#1204)

The `fusedDistanceNNMinReduce` is not documented or exposed downstream, so removing it. This reduced libcuvs.so size by ~0.5 MB, when I compiled for single arch.

Authors:
  - Vinay Deshpande (https://github.com/vinaydes)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: rapidsai#1204

Author: vinaydes

cpp/CMakeLists.txt

@@ -377,7 +377,6 @@ if(BUILD_SHARED_LIBS)
     src/distance/detail/pairwise_matrix/dispatch_rbf.cu
     src/distance/detail/pairwise_matrix/dispatch_l2_expanded_double_double_double_int64_t.cu
     src/distance/detail/pairwise_matrix/dispatch_l2_expanded_float_float_float_int64_t.cu
-    src/distance/detail/fused_distance_nn.cu
     src/distance/distance.cu
     src/distance/pairwise_distance.cu
     src/distance/sparse_distance.cu