Revert "generate: Add fallback for missing "since""

This reverts commit 4fa286d5.

Turns out that a couple libraries (cough WebKitGTK cough) don't match the namespace version with their library version, so the fallback and the availability become absolute nonsense.

The proper fix is to have the template handle the fallback using the configuration to determine the base version that introduced a symbol. Or those libraries stop trying to be too clever for their own good.

Fixes: #179 (closed)