configure: remove block on POWER 7/BE systems

We thought there was a silent data corruption issue on POWER 7/BE
systems, so we blocked building on POWER 7/BE systems altogether.  We
later figured out that it was just data hangs -- not silent data
corruption.  So in hindsight, the configure block probably wasn't
necessary -- but we didn't know it at the time.

Regardless, the hangs have now been fixed, and we're removing the
POWER 7/BE block in configure.

For more detail on the entire saga, see
https://github.com/open-mpi/ompi/issues/4349#issuecomment-374970982.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

configure.ac

@@ -84,17 +84,6 @@ AS_IF([test "$host" != "$target"],
       [AC_MSG_WARN([Cross-compile detected])
        AC_MSG_WARN([Cross-compiling is only partially supported])
        AC_MSG_WARN([Proceed at your own risk!])])
-# Check for architectures that we explicitly no longer support
-case "${host}" in
-    powerpc-*|powerpc64-*|ppc-*)
-        AC_MSG_ERROR([Big endian PPC is no longer supported.])
-        ;;
-esac
-case "${target}" in
-    powerpc-*|powerpc64-*|ppc-*)
-        AC_MSG_ERROR([Big endian PPC is no longer supported.])
-        ;;
-esac
 
 # AC_USE_SYSTEM_EXTENSIONS alters CFLAGS (e.g., adds -g -O2)
 OPAL_VAR_SCOPE_PUSH([CFLAGS_save])