reduce size: remove lib/libgdal.a

[visr]

Quoting @tshort from discourse https://discourse.julialang.org/t/package-size-crazziness/14644/18

In the Win64 GDAL library built by GDALBuilder, an easy way to trim size is to remove lib/libgdal.a (159 MB out of 314 MB).

[tshort]

Just to confirm this, I asked if there's any reason not to delete lib/*.a files on the bindeps2 Slack channel. Elliot responded:

Only if you want to support someone else compiling against the static library in the future.
Feel free to delete that within your buildscript if that isn’t part of what you want to support.

Also, this isn't just Windows. All platforms include lib/*.a files.

[visr]

Ok, thanks for checking! I don't see how or why to support that now, so I guess this would be good to remove.