MAINT remove .c files from .gitattributes

.c files haven't been under version control for some time now.

.gitattributes

@@ -1,31 +1 @@
-/sklearn/__check_build/_check_build.c -diff
-/sklearn/_isotonic.c -diff
-/sklearn/cluster/_dbscan_inner.cpp -diff
-/sklearn/cluster/_hierarchical.cpp -diff
-/sklearn/cluster/_k_means.c -diff
-/sklearn/cluster/_k_means_elkan.c -diff
-/sklearn/datasets/_svmlight_format.c -diff
-/sklearn/decomposition/_online_lda.c -diff
-/sklearn/decomposition/cdnmf_fast.c -diff
-/sklearn/ensemble/_gradient_boosting.c -diff
-/sklearn/feature_extraction/_hashing.c -diff
-/sklearn/linear_model/cd_fast.c -diff
-/sklearn/linear_model/sgd_fast.c -diff
-/sklearn/linear_model/sag_fast.c -diff
-/sklearn/metrics/pairwise_fast.c -diff
-/sklearn/neighbors/ball_tree.c -diff
-/sklearn/neighbors/kd_tree.c -diff
-/sklearn/svm/liblinear.c -diff
-/sklearn/svm/libsvm.c -diff
-/sklearn/svm/libsvm_sparse.c -diff
-/sklearn/tree/_tree.c -diff
-/sklearn/tree/_utils.c -diff
-/sklearn/utils/arrayfuncs.c -diff
-/sklearn/utils/graph_shortest_path.c -diff
-/sklearn/utils/lgamma.c -diff
-/sklearn/utils/_logistic_sigmoid.c -diff
-/sklearn/utils/murmurhash.c -diff
-/sklearn/utils/seq_dataset.c -diff
-/sklearn/utils/sparsefuncs_fast.c -diff
-/sklearn/utils/weight_vector.c -diff
 /doc/whats_new.rst merge=union