Drop I2P support with libtorrent 1.2

[glassez]

libtorrent 1.2 does not contain a lot of I2P related fixes made recently, and I2P is almost completely unusable there (according to the feedbacks of the users involved).

Also fixes build when libtorrent is compiled with I2P support disabled.
Closes #19152.

[glassez]

@Vort, ping!

[xavier2k6]

@glassez I haven't been able to test I2P myself but there are other issues with I2P support that need to be handled as well (when possible)....quick summary -> #19066 (comment)

[Vort]

I did not tested I2P support with 1.2 and do not know what users think about it, but I plan to check 2.0 support for regressions once build is fixed.

[Pentaphon]

libtorrent 1.2 does not contain a lot of I2P related fixes made recently, and I2P is almost completely unusable there (according to the feedbacks of the users involved).

As long as there continues to be libtorrent 1.2 qBit builds (since arvid is still pushing out updates to 1.2.x) then it should be fine. People should be notified that they need to use 2.0.x builds to get i2p support, which admittedly, not a lot of people care about yet. I don't think most people even know what i2p even is.

libtorrent 1.2 builds should remain default until 2.x becomes as stable as 1.x.

[glassez]

I did not tested I2P support with 1.2 and do not know what users think about it

As per #19066:

In, the realease, 4.6.0alpha1 (lt12 qt6) I2P does not work at all - neither via I2PD nor via JAVA I2P

[Vort]

I checked 2.0 build from this branch: I2P download and upload works ok.

[glassez]

there are other issues with I2P support that need to be handled as well (when possible)....quick summary -> #19066 (comment)

I am aware of this. However, it is not a priority for me, although it is on my to-do list. I'll come back to it from time to time.