libprocessgroup: Remove __BEGIN_DECLS and __END_DECLS

These macros have been useless in this file since commit 82b72a56
("libprocessgroup: Add support for task profiles") in 2018 which added
C++ includes, and C++ types and keywords inside an extern "C" block.

Change-Id: Iae1cbfcc5cb974034d88fb466b79088496190940