TT#6431 Revert "TT#6431 Build-Depend on either default-libmysqlclient-dev or libmysql++-dev"

This reverts commit 693429e353.

libmysql++-dev is needed for building, it was just hidden by the
broken sems build system and therefore commit 693429e353 looked
like it was ok but it clearly wasn't (the mysql++/*.h files provided
by libmysql++-dev are required).

FTR: libmysql++-dev is on its way back to Debian testing (AKA stretch)
currently.

Change-Id: I1d7ab3ea0c3013f6fedb3f19a9d8b945d20fb2e0

debian/control

@@ -3,12 +3,12 @@ Section: net
 Priority: optional
 Maintainer: Sipwise Development Team <support@sipwise.com>
 Build-Depends: debhelper (>= 8~),
-               default-libmysqlclient-dev | libmysql++-dev,
                flite-dev,
                libcurl4-openssl-dev | libcurl4-gnutls-dev,
                libev-dev,
                libevent-dev (>= 2.0.0),
                libhiredis-dev,
+               libmysql++-dev,
                libopus-dev,
                libpcre3-dev,
                libspandsp-dev,