Remove last usage of StrictSubversionDownloadStrategy

This strategy is only used in one formula, so we can just replace it
with a dependency. It could be done with a requirement, but since this
is a low-impact change (it only matters for head builds on Leopard), I'm
not sure it's worth the effort.

The constant is retained for compatibility, though I could find only one
other reference to it on GitHub, in a formula that was removed from core
almost two years ago.