Remove legacy code from D7

It's quite possible that nothing from what's left in "legacy" is relevant anymore for the D8+ version of Feeds Tamper. I don't know exactly what every function does or is used for. Or at least I didn't put the effort in yet to read and understand it.

Some functions do look related to make safe urls for editing a Tamper for a source. In the D8+ version UUID's are used to make safe urls.

For example, I don't know where pack('H*', $source) is exactly useful for.

Ah, that makes sense if the code is about generating safe URLs.

It certainly seems like there's nothing left useful-- there's a general include file, but all the code it supported is gone, and then the UI submodule, but the 8.x branch already has a working UI.