Remove unused function from executorch/backends/apple/coreml/runtime/inmemoryfs/inmemory_filesystem.cpp (#4240)

Summary:
Pull Request resolved: #4240

`-Wunused-function` has identified an unused function. This diff removes it. In many cases these functions have existed for years in an unused state.

Reviewed By: palmje

Differential Revision: D59644712

fbshipit-source-id: 7dc416d17ba8acf7251e8c60cf6e2abe2cf571a4

Author: r-barnes

backends/apple/coreml/runtime/inmemoryfs/inmemory_filesystem.cpp

@@ -115,12 +115,6 @@ InMemoryFileSystem::InMemoryNode* get_node(InMemoryFileSystem::InMemoryNode* nod
     return node;
 }
 
-std::string toString(time_t time) {
-    constexpr auto format = "%Y-%m-%dT%TZ";
-    std::stringstream stream;
-    stream << std::put_time(gmtime(&time), format);
-    return stream.str();
-}
 
 time_t toTime(const std::string& str) {
     constexpr auto format = "%Y-%m-%dT%TZ";