Remove -debug-time-compilation

This flag no longer does anything now that the unified statistics
reporting infrastructure exists. It is better to use
-driver-time-compilation to see a bird's eye view of timing statistics
for frontend jobs, and -stats-output-dir to see a down-and-dirty view of
everything including performance counters.

lib/FrontendTool/FrontendTool.cpp

@@ -2232,9 +2232,6 @@ int swift::performFrontend(ArrayRef<const char *> Args,
   PDC.setFormattingStyle(
       Invocation.getDiagnosticOptions().PrintedFormattingStyle);
 
-  if (Invocation.getFrontendOptions().DebugTimeCompilation)
-    SharedTimer::enableCompilationTimers();
-
   if (Invocation.getFrontendOptions().PrintStats) {
     llvm::EnableStatistics();
   }