ClangTidy fixes

Change-Id: Id7358fcdccae69b46b479d55ed6aee80efd3431e

internal/ceres/solver.cc

diff --git a/internal/ceres/solver.cc b/internal/ceres/solver.cc
index 010ad26..97c1d87 100644
--- a/internal/ceres/solver.cc
+++ b/internal/ceres/solver.cc
@@ -163,9 +163,6 @@
       "was not enabled when Ceres Solver was built.";
   constexpr char kNoNesdisFormat[] =
       "NESDIS is not available with sparse_linear_algebra_library_type = %s.";
-  constexpr char kMixedFormat[] =
-      "use_mixed_precision_solves with %s is not supported with "
-      "sparse_linear_algebra_library_type = %s";
   constexpr char kDynamicSparsityFormat[] =
       "dynamic sparsity is not supported with "
       "sparse_linear_algebra_library_type = %s";