Remove the "small" size designation for thread_identity_test, which causes the test to timeout after 60s.

Motivation: this test times out flakily in some build configurations.
PiperOrigin-RevId: 758364551
Change-Id: Ic988ec988ddb51439875cba30b137348c59457ee

Author: ezbr

absl/base/BUILD.bazel

@@ -685,7 +685,6 @@ cc_test(
 
 cc_test(
     name = "thread_identity_test",
-    size = "small",
     srcs = ["internal/thread_identity_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,