[m-rev.] for review: Delete unused MR_CAN_GET_PC_AT_SIGNAL macro.

[Julien Fischer]

On Tue, 13 Oct 2020, Peter Wang wrote:

> runtime/mercury_conf_param.h:
>    As above.

That's fine; it doesn't look as though it's ever been used
since being added in 1998.

Julien.