Remove suggestion that add_global_mapping() is unused

[gmarkall]

It is used in various places in Numba. The comment here forces the reader to do a little pointless investigation, so remove it.

[esc]

@stuartarchibald I think this is in the wrong milestone. 0.41.1 was already released on 17th Oct, so punting this to 0.42.0.

[stuartarchibald]

Ah yes, thanks @esc.