Drop std:: on size_t in various spots. (#4546)

We predominantly omit the `std::` in these cases already. This is for
style: "Prefer to omit the std:: prefix for these types, as the extra 5
characters do not merit the added clutter."
(https://google.github.io/styleguide/cppguide.html#Integer_Types)

toolchain/parse/parse_fuzzer.cpp

@@ -15,8 +15,7 @@
 namespace Carbon::Testing {
 
 // NOLINTNEXTLINE: Match the documented fuzzer entry point declaration style.
-extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data,
-                                      std::size_t size) {
+extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data, size_t size) {
   // Ignore large inputs.
   // TODO: See tokenized_buffer_fuzzer.cpp.
   if (size > 100000) {