Remove getEstimatedClockResolution from Timer

It used to be part of the experimental benchmarking support, but
since that was replaced with proper benchmarking support with its
own timer facilities, it is now a dead code and useless.

src/catch2/catch_timer.hpp

@@ -13,7 +13,6 @@
 namespace Catch {
 
     auto getCurrentNanosecondsSinceEpoch() -> uint64_t;
-    auto getEstimatedClockResolution() -> uint64_t;
 
     class Timer {
         uint64_t m_nanoseconds = 0;