Bug 42747: Discard unsupported updates earlier.

Firefox's updater has a function to select updates, which checks mainly
the version number.
Therefore, a more recent update that is unsupported will be chosen over
a compatible one.
We patch this to be able to provide an alternative update path to
Windows 7.

Revert "Bug 19121: reinstate the update.xml hash check"

This reverts commit 39a712fb.