`overload_test`: Remove a few unnecessary trailing return types

`absl::StrCat` always returns a `std::string`. Specifying it using a trailing return type does not help.
Tests are documentation, and they should be consistent with [the google style guide](https://google.github.io/styleguide/cppguide.html#trailing_return).

For example, this is different than `-> absl::string_view` and returning string literals, where it is actually a load-bearing change.

PiperOrigin-RevId: 636705683
Change-Id: I0d84a562a59bc0c16be01dd2ae5538adb401432e

absl/functional/overload_test.cc

diff --git a/absl/functional/overload_test.cc b/absl/functional/overload_test.cc
index a2e8767..92f0eb9 100644
--- a/absl/functional/overload_test.cc
+++ b/absl/functional/overload_test.cc
@@ -32,12 +32,10 @@
 
 TEST(OverloadTest, DispatchConsidersTypeWithAutoFallback) {
   auto overloaded = absl::Overload{
-      [](int v) -> std::string { return absl::StrCat("int ", v); },
-      [](double v) -> std::string { return absl::StrCat("double ", v); },
-      [](const char* v) -> std::string {
-        return absl::StrCat("const char* ", v);
-      },
-      [](auto v) -> std::string { return absl::StrCat("auto ", v); },
+      [](int v) { return absl::StrCat("int ", v); },
+      [](double v) { return absl::StrCat("double ", v); },
+      [](const char* v) { return absl::StrCat("const char* ", v); },
+      [](auto v) { return absl::StrCat("auto ", v); },
   };
 
   EXPECT_EQ("int 1", overloaded(1));