ASoC: wm8991: Remove unused pointer in wm8991_probe()

Remove unused pointer 'wm8991' in function wm8991_probe(). The last user
vanished with a86652e5 (ASoC: wm8991: Convert to direct regmap API usage)

Detected by Coverity: CID 1162831

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