ceres/stringprintf -> absl::strings

Replace ceres::String* with their more modern and performant
absl strings library equivalent and delete our string
manipulation library.

Change-Id: Iecbdba9864e0abf329778f81fdc0708f78f7594f

internal/ceres/function_sample.cc

@@ -30,7 +30,7 @@
 
 #include "ceres/function_sample.h"
 
-#include "ceres/stringprintf.h"
+#include "absl/strings/str_format.h"
 
 namespace ceres::internal {
 
@@ -64,7 +64,7 @@ FunctionSample::FunctionSample(const double x,
       gradient_is_valid(true) {}
 
 std::string FunctionSample::ToDebugString() const {
-  return StringPrintf(
+  return absl::StrFormat(
       "[x: %.8e, value: %.8e, gradient: %.8e, "
       "value_is_valid: %d, gradient_is_valid: %d]",
       x,