Remove unused private field freeDiskSpaceKB

Static analysis identified an unused private
field freeDiskSpaceKB in the EncoderLink class.
The usage was remove in commit a4796b5fc9
'Storage Groups (a novel by Chris Pinkham)',
but the variable remained.  Remove it from
the class and ctors

Fixes #11802

Signed-off-by: Stuart Morgan <smorgan@mythtv.org>