GNUmakefile.in defined a macro "garbage" that seems to have been meant
as a suitable target for automatic "rm -rf" treatment, but it isn't
actually used anywhere (and indeed never was, AFAICT).

Moreover, we have concluded that the Makefiles shouldn't take it upon
themselves to remove files that aren't expected by-products of building,
so that doing anything like that would be against project policy anyway.
Hence, just remove the macro.

Grepping around finds another violation of that policy in ecpg/preproc,
so clean that up too.

Daniel Gustafsson (ecpg change by me)

Discussion: https://postgr.es/m/AFBEF63E-E19D-4EBB-9F08-4617CDC751ED@yesql.se