mmc: host: delete obsolete sdhci-pxa.c

Delete obsolete sdhci-pxa.c, which was previously shared amongst the
entire PXA series.  Instead we now use sdhci-pxav3.c for mmp2 and
sdhci-pxav2.c for pxa9xx.

Signed-off-by: Zhangfei Gao <zhangfei.gao@marvell.com>
Acked-by: Philip Rakity <prakity@marvell.com>
Acked-by: Mark F. Brown <mark.brown314@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>