Fix Debian bug #985763

Bastian Germann reports in #985763:

code/back_end-hsm contains doc/HSM-CST_UG.pdf which has an unclear license and is probably not in source format. Also, the OASIS PKCS#11 headers are contained in the subdirectory src/include. It is unclear whether OASIS is DFSG-free (see #952951).

As the package is not built with the hsm backend, an easy solution is removing it entirely.

I have done the change in Debian and uploaded it already.

cc @d4s — you worked with this tool, it’d be great if you could confirm it’s okay (to the best of my knowledge the HSM backend was never compiled in, but just in case).