Remove unnecessary dummy_dep

meson.override_find_program() has existed since 0.46. In order to use gi-docgen as a subproject now, use the following in a wrap file.

[provide]
program_names=gi-docgen

Then a consumer of gi-docgen just needs to use:

gi_docgen = find_program('gi-docgen')

If gi-docgen is not found on the system, then the gi-docgen wrap will be triggered.

Note that it was already possible to use gi-docgen in this way, but this should push consumers into better Meson patterns.