Before bug 938437, we had a rather large and error-prone
nsStaticXULComponents.cpp used to register all modules. That was
replaced with clever use of the linker, which allowed to avoid the mess
that maintaining that file was.
Fast forward to now, where after bug 1524687 and other work that
preceded it, we have a much smaller number of remaining static xpcom
components, registered via this linker hack, and don't expect to add
any new ones. The list should eventually go down to zero.
Within that context, it seems to be the right time to get rid of the
magic, and with it the problems it causes on its own.
Some of those components could probably be trivially be converted to
static registration via .conf files, but I didn't want to deal with the
possible need to increase the number of dummy modules in XPCOMInit.cpp.
They can still be converted as a followup.