add 70-no-bitmaps-except-emoji.conf in postrm for removal

Dear Maintainer,

In the commit (c6256046) they forgot to include the removal of the file 70-no-bitmaps-except-emoji.conf in postrm.

I tested the xterm package with piuparts and saw this errors for fontconfig:

1m24.8s ERROR: WARN: Broken symlinks:
/etc/fonts/conf.d/70-no-bitmaps-except-emoji.conf -> /usr/share/fontconfig/conf.avail/70-no-bitmaps-except-emoji.conf
1m25.4s ERROR: FAIL: Package purging left files on system:
/etc/fonts/ owned by: fontconfig-config, fonts-dejavu-core, fonts-dejavu-mono
/etc/fonts/conf.d/ owned by: fontconfig-config, fonts-dejavu-core, fonts-dejavu-mono
/etc/fonts/conf.d/70-no-bitmaps-except-emoji.conf -> /usr/share/fontconfig/conf.avail/70-no-bitmaps-except-emoji.conf not owned

my commit fixes it.

after:

$ patch -p1 < ../postrm-fix.patch
$ dpkg-buildpackage -b -uc
...
-> piuparts tests good

Thanks!