ASoC: wm8993: Remove unused pointer in wm8993_remove()

Commit 88b5bdfd (ASoC: wm8993: drop regulator_bulk_free of devm_ allocated
data) eliminated the last user of driver data pointer 'wm8993' in function
wm8993_remove() - Thus remove it. Detected by Coverity: CID 1186208.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: Mark Brown <broonie@linaro.org>