delete unused String.h/cpp

Summary: Nobody has used these since they were added 1+ year ago. Deleting for now.

Reviewed By: mergennachin

Differential Revision: D47272109

fbshipit-source-id: d6caea66295af2dd38c1d25e75ca932e212761fc

Author: JacobSzwejbka

core/String.cpp

@@ -9,16 +9,12 @@ def define_common_targets():
 
     runtime.cxx_library(
         name = "core",
-        srcs = [
-            "String.cpp",
-        ],
         exported_headers = [
             "ArrayRef.h",
             "Constants.h",
             "Error.h",
             "FunctionRef.h",
             "Result.h",
-            "String.h",
             "macros.h",
             "span.h",
         ],