Remove a duplicated field in SWelsSvcCodingParam

The same field already exists in SEncParamExt. When the
initialization was deduplicated in ac404ce, this lead to
reading the now uninitialized field in SWelsSvcCodingParam,
instead of the properly initialized one in the base class.
Merge pull request #1516 from mstorsjo/remove-duplicated-field

Remove a duplicated field in SWelsSvcCodingParam