Don't preserve Host header during Apache proxying

When Apache proxy forwards a request it usually makes the request to the
backend server with a new Host: header but sends the original host
information using a special header X-FORWARDED-HOST: header.  It is a
common practise.  The backend can be make the choice of which header to
use based on the knowledge that it is running behind a proxy server.

Since hostname of a FreedomBox machine could be anyting and it could
change during the operation of Plinth, having a list of incoming Host:
header values is not going to be easy in Plinth.  If Apache were not to
force override this header, managing list of Host: header values will
become easiler in Plinth.  If Plinth indeed wants to know the original
host name by which user refferred to it, it can use the
X-FORWARDED-HOST: header which is the norm in these situtations.

data/etc/apache2/sites-available/plinth-ssl.conf

@@ -9,7 +9,6 @@
     Header always set Strict-Transport-Security "max-age=31536000"
 
     ## Shared options.
-    ProxyPreserveHost on
     DocumentRoot /usr/share/plinth
 
     ## Use this rule to hang plinth off of plinth.(servername)