configure: Drop dead code attempting to use -msmall-data on alpha hosts

In commit 823eb013 we moved the setting of ARCH from configure
to meson.build, but we accidentally left behind one attempt to use
$ARCH in configure, which was trying to add -msmall-data to the
compiler flags on Alpha hosts.  Since ARCH is now never set, the test
always fails and we never add the flag.

There isn't actually any need to use this compiler flag on Alpha:
the original intent was that it would allow us to simplify our TCG
codegen on that platform, but we never actually made the TCG changes
that would rely on -msmall-data.

Drop the effectively-dead code from configure, as we don't need it.

This was spotted by shellcheck:

In ./configure line 2254:
case "$ARCH" in
      ^---^ SC2153: Possible misspelling: ARCH may not be assigned, but arch is.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20220720152631.450903-5-peter.maydell@linaro.org