Thunderbird prefers[1] directory-rel for account local paths and only
falls back to directory when the relative pref is absent. To add, the
generated directory values were home-relative paths while the app
expects absolute paths when reading the setting. Instead of fixing
these, just drop the redundant settings.
[1] https://searchfox.org/comm-central/source/mailnews/base/src/nsMsgIncomingServer.cpp#745
