Finish removal of ia32 Windows builds

[Half-Shot]

We started the process 1 and a bit years ago in element-hq/element-desktop#1666, and now I think we've given enough time to end support for it.

[t3chguy]

@Half-Shot we already don't provide ia32/win32 builds? We just run it in CI because electron-builder still supports it, so someone may still be building them