ALSA: compress: Remove unused variable

Commit 04c5d5a4 ("ALSA: compress: Embed struct device") removed
the statement that used 'str' but didn't remove the variable itself.
So remove it.

[Adding stable to Cc since pr_debug() may refer to the uninitialized
 buffer -- tiwai]

Fixes: 04c5d5a4 ("ALSA: compress: Embed struct device")
Signed-off-by: Guneshwor Singh <guneshwor.o.singh@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>