src/util/source_location.h - `set_function` and `get_function`

[NlightNFotis]

Remove methods source_locationt::set_function and source_locationt::get_function marked DEPRECATED. Expected to be relatively hard as the methods are pervasively used inside of the codebase.

[NlightNFotis]

The following two PRs have (partially?) dealt with the issue outlined above:

• Remove function from source_locationt [depends-on: #4131] #3514
• label_properties: do not rely on source_locationt's get_function() #4131