Revert "Declare em-filter-i18n.h as BUILT_SOURCES in mail/Makefile.am"

It turned out that the file is required for translators, which
are not supposed to build sources, thus the built file would
be missing to them. I wasn't aware of that.

This reverts commit fa75665c3bf9dceaeec233ccf3702cdc167de3c3.