Bug 1674045 - Remove unused ProfilerStringView::String() - r=gregtatum

The previous patch removed the only two uses of ProfilerStringView::String().
Since it can be potentially expensive (creating a `std::string` object, sometimes allocating a buffer, and copying the string contents), it's best to remove it completely.

Differential Revision: https://phabricator.services.mozilla.com/D95116

mozglue/baseprofiler/public/BaseProfilerMarkersPrerequisites.h

@@ -186,10 +186,6 @@ class MOZ_STACK_CLASS ProfilerStringView {
     return Span<const char>(Data(), Length());
   }
 
-  [[nodiscard]] std::basic_string<CHAR> String() const {
-    return std::basic_string<CHAR>(mStringView);
-  }
-
  private:
   enum class Ownership { Literal, Reference, OwnedThroughStringView };