Remove redundant const of ThreadLocal #1597

Job	Run time
gcc-compile-with-boringssl	18m 23s
gcc-compile-with-bazel-all-options	17m 56s
clang-compile-with-make	3m 6s
gcc-compile-with-bazel	17m 0s
gcc-compile-with-cmake	3m 16s
gcc-compile-with-cmake-all-options	6m 6s
gcc-compile-with-make	2m 47s
gcc-compile-with-make-all-options	5m 12s
clang-compile-with-bazel	15m 48s
clang-compile-with-bazel-all-options	16m 34s
clang-compile-with-boringssl	16m 51s
clang-compile-with-cmake	2m 52s
clang-compile-with-cmake-all-options	5m 3s
clang-compile-with-make-all-options	2m 54s
clang-unittest	8m 20s
	2h 22m 8s

Provide feedback