cmake: Lets do not explicitely set a APPLICATION_SHORTNAME.

That makes it defaulting to the APPLICATION_NAME which is ownCloud.
That is what we want and expect to read the system exclude file from
on linux.

How we find the system exclude file linux:

ConfigFile::excludeFileFromSystem()
 -> Theme::instance()->appName()

 -> OwncloudTheme::appName() returns "ownCloud"

 -> default Theme::appName() returns APPLICATION_SHORTNAME

 -> cmake installs to ${SYSCONFDIR}/${APPLICATION_SHORTNAME}

OWNCLOUD.cmake

@@ -1,5 +1,4 @@
 set( APPLICATION_NAME       "ownCloud" )
-set( APPLICATION_SHORTNAME  "owncloud" )
 set( APPLICATION_EXECUTABLE "owncloud" )
 set( APPLICATION_DOMAIN     "owncloud.com" )
 set( APPLICATION_VENDOR     "ownCloud" )