Loolwsd proof support collabora 4.2.x

[raffis]

Describe the change

Loolowsd has now support for proof key validation. Looks like this was committed in December 2019.

Current situation

No proof validation for loolwsd.

Should

Looks like a proof key needs to be generated manually.
Remove the skip proof validation for loolwsd.

Additional context

libreoffice-convert_1  | wsd-00029-00029 2020-01-16 12:15:10.181204 [ loolwsd ] INF  Registering filesystem for space checks: [/opt/lool/child-roots/.]| common/FileUtil.cpp:260
libreoffice-convert_1  | wsd-00029-00029 2020-01-16 12:15:10.181240 [ loolwsd ] DBG  FileServerRoot: /usr/share/loolwsd| wsd/LOOLWSD.cpp:3389
libreoffice-convert_1  | Could not find /etc/loolwsd/proof_key
libreoffice-convert_1  | No proof-key will be present in discovery.
libreoffice-convert_1  | Generate an RSA key using this command line:
libreoffice-convert_1  |     ssh-keygen -t rsa -N "" -f "/etc/loolwsd/proof_key"
libreoffice-convert_1  | wsd-00029-00029 2020-01-16 12:15:10.182591 [ loolwsd ] WRN  Could not find /etc/loolwsd/proof_key
libreoffice-convert_1  | No proof-key will be present in discovery.
libreoffice-convert_1  | Generate an RSA key using this command line:
libreoffice-convert_1  |     ssh-keygen -t rsa -N "" -f "/etc/loolwsd/proof_key"| wsd/ProofKey.cpp:140