Get rid of traces of the old-style ALL_MOBJS variable.

This also fixes the following issue, which could cause a warning with
certain configs:

Makefiles (legally) do:

obj-$(CONFIG_SND_AD1848) += snd-pcm.o snd-timer.o snd.o
obj-$(CONFIG_SND_CS4231) += snd-pcm.o snd-timer.o snd.o snd-rawmidi.o

so when the first option is set to y, the second to m, we have e.g.
snd-pcm.o on both $(obj-y) and $(obj-m). We correctly don't build
the modular version in this case.

However, if snd-pcm.o was a multi-part object, we did automatically
generate a link rule for both the modular and the built-in version, 
which caused a warning - that's now fixed.