benchmark: remove duplicate ffi string length direct

[HoonDongKang]

The string_length FFI benchmark had four cases:

• string-length-string.js passes the JavaScript string "hello".
• string-length-buffer.js passes Buffer.from('hello\0').
• string-length-string-direct.js and string-length-buffer-direct.js both pass a raw pointer obtained with ffi.getRawPointer().

The two direct benchmarks contained identical code: both created the same Buffer, obtained its raw pointer, and passed that pointer to string_length. This PR replaces them with a single string-length-direct.js benchmark so the same case runs only once.

[nodejs-github-bot]

Review requested:

• @nodejs/performance