build: remove unused cpp library rt

[shoothzj]

Motivation

Fix build warnings, and librt is not used.

[INFO] --- nar-maven-plugin:3.5.2:nar-compile (default-nar-compile) @ circe-checksum ---
[INFO] Compiling 2 native files
[WARNING] No file matching patterns ("librt.a, librt.so) for library name "rt" was found.
[INFO] 2 total files to be compiled.

[shoothzj]

@dlg99 @eolivelli @hangc0276 @merlimat @nicoloboschi @StevenLuMT @wenbingshen @zhaijack PTAL