ALSA: memalloc: Drop Xen PV workaround again

Since recently in the commit e469e204 ("ALSA: memalloc: Let IOMMU
handle S/G primarily"), the SG buffer allocation code was modified to
use the standard DMA code primarily and the fallback is applied only
limitedly.  This made the Xen PV specific workarounds we took in the
commit 53466ebd ("ALSA: memalloc: Workaround for Xen PV") rather
superfluous.

It was a hackish workaround for the regression at that time, and it
seems that it's causing another issues (reportedly memory
corruptions).  So it's better to clean it up, after all.

Link: https://lore.kernel.org/20240906184209.25423-1-ariadne@ariadne.space
Cc: Ariadne Conill <ariadne@ariadne.space>
Link: https://patch.msgid.link/20240910113100.32542-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>