thunderbird-bin: remove GConf dependency

It is only optionally dlopened by crash reporter to try to get system proxy settings
but no system stores proxy in GConf these days so it is completely useless.

https://github.com/mozilla/gecko-dev/blob/97c590ed5532a7b382b5b8500c109de8f1d831e3/toolkit/crashreporter/client/crashreporter_gtk_common.cpp#L121-L122