remove useless `__buildtin_unreachable()` usage

[pthierry-ledger]

The compiler's __builtin_unreachable() is made when a given path is never reached, while, in the same time, no noreturn usage is explicitely defined above. This builtin is used only in order to avoid compiler's reach the end of non void fonction warning.

In our case, when using panic(), the noreturn is explicitly declared in the panic system, and do not requires the usage of __builtin_unreachable(). As a consequence, the builtin calls should be removed, as they are useless in the panic case.