Drop the g++ build-dep, transition is done

It's not cross-satisfiable.

Reported-by: Helmut Grohne

debian/control

@@ -9,8 +9,7 @@ Build-Depends: dpkg-dev (>= 1.17.14), debhelper (>= 9.20141010), libdb-dev,
  gettext (>= 0.12), libcurl4-gnutls-dev (>= 7.19.4~),
  zlib1g-dev, libbz2-dev, liblzma-dev, liblz4-dev (>= 0.0~r126),
  xsltproc, docbook-xsl, docbook-xml, po4a (>= 0.34-2),
- autotools-dev, autoconf, automake, libgtest-dev <!nocheck>,
- g++ (>= 4:5.2)
+ autotools-dev, autoconf, automake, libgtest-dev <!nocheck>
 Build-Depends-Indep: doxygen, w3m, graphviz
 Build-Conflicts: autoconf2.13, automake1.4
 Vcs-Git: git://anonscm.debian.org/apt/apt.git