Remove CATCH_CONFIG_ENABLE_BENCHMARKING compilation toggle

Now that Catch2 is a proper library, we can always build the full
library (comparatively minor slowdown) and the user can avoid
including benchmarking headers to avoid the compilation slowdown.

docs/benchmarks.md

@@ -3,10 +3,6 @@
 
 > [Introduced](https://github.com/catchorg/Catch2/issues/1616) in Catch 2.9.0.
 
-_Note that benchmarking support is disabled by default and to enable it,
-you need to define `CATCH_CONFIG_ENABLE_BENCHMARKING`. For more details,
-see the [compile-time configuration documentation](configuration.md#top)._
-
 Writing benchmarks is not easy. Catch simplifies certain aspects but you'll
 always need to take care about various aspects. Understanding a few things about
 the way Catch runs your code will be very helpful when writing your benchmarks.

docs/configuration.md

@@ -152,7 +152,6 @@ by using `_NO_` in the macro, e.g. `CATCH_CONFIG_NO_CPP17_UNCAUGHT_EXCEPTIONS`.
     CATCH_CONFIG_DISABLE                    // Disables assertions and test case registration
     CATCH_CONFIG_WCHAR                      // Enables use of wchart_t
     CATCH_CONFIG_EXPERIMENTAL_REDIRECT      // Enables the new (experimental) way of capturing stdout/stderr
-    CATCH_CONFIG_ENABLE_BENCHMARKING        // Enables the integrated benchmarking features (has a significant effect on compilation speed)
     CATCH_CONFIG_USE_ASYNC                  // Force parallel statistical processing of samples during benchmarking
     CATCH_CONFIG_ANDROID_LOGWRITE           // Use android's logging system for debug output
     CATCH_CONFIG_GLOBAL_NEXTAFTER           // Use nextafter{,f,l} instead of std::nextafter