Disable Firefox's suffix and prefix adding in I2P Browser

This feature—while useful on the internet—doesn’t help at all on I2P since all sites end in .i2p.

I’ve not seen the autoguessing happen during normal use on Tails but I have seen it while working on the test suite.

Feature Branch: kytv:bug/10184-do_not_set_hostname_prefix_or_suffix_in_I2PBrowser

Original created by @kytv on 10184 (Redmine)

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