modules: remove standalone AM_CFLAGS usage

It's implied when the xxx_CFLAGS/xxx_CPPFLAGS is not set.

By default the values for AM_CFLAGS is are

AM_CFLAGS = \
	-avoid-version -module \
	-export-symbols-regex ^vlc_entry \
	-shrext $(LIBEXT) \
	-no-undefined \
	$(top_builddir)/compat/libcompat.la $(LTLIBVLCCORE)

A lot of modules don't set it and are working just fine.

Ref. !929 (merged)