alpha-softmmu: Disable for the 0.15 release branch.

The system emulation code was not merged before the branch.
Let's leave that work for the next release.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/configure b/configure
index 8546573..fc60888 100755
--- a/configure
+++ b/configure
@@ -846,7 +846,6 @@
     default_target_list="\
 i386-softmmu \
 x86_64-softmmu \
-alpha-softmmu \
 arm-softmmu \
 cris-softmmu \
 lm32-softmmu \