Drop deprecated sip4 from Build-Depends

On Debian/wheezy there's:

| # apt-cache show sip4
| Package: sip4
| [...]
| Version: 4.10.2-1
| Depends: python-sip-dev

and since we're build-depending on python-sip-dev already
there's no point in having sip4 in Build-Depends any longer,
since sip4 is a *source* only package starting with Debian/wheezy
and threfore doesn't work as build-dependency anyway.

debian/control

@@ -14,8 +14,7 @@ Build-Depends: debhelper (>= 7),
                libxml2-dev,
                openssl,
                python-dev,
-               python-sip-dev,
-               sip4
+               python-sip-dev
 Standards-Version: 3.9.3
 
 Package: ngcp-sems