src: inline `SLICE_START_END()` in node_buffer.cc

This macro is only used once, so it doesn’t need to be a macro.

PR-URL: https://github.com/nodejs/node/pull/29357


Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>