[turbofan] Remove dead (and scary) {OsrHelper} constructor.

R=neis@chromium.org

Change-Id: I23298e2c0adcfdc4e6e963e98cde641bef9cdb5b
Reviewed-on: https://chromium-review.googlesource.com/539296Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45996}

src/compiler/osr.h

@@ -92,10 +92,6 @@ class Linkage;
 class OsrHelper {
  public:
   explicit OsrHelper(CompilationInfo* info);
-  // Only for testing.
-  OsrHelper(size_t parameter_count, size_t stack_slot_count)
-      : parameter_count_(parameter_count),
-        stack_slot_count_(stack_slot_count) {}
 
   // Deconstructs the artificial {OsrNormalEntry} and rewrites the graph so
   // that only the path corresponding to {OsrLoopEntry} remains.