Do not set unhonored DPKG_NO_TSTP variable for dpkg

Support for that variable was removed in dpkg in 1.15.6, in commit
6f037003e8b96878b485efb7cbd1f846e3bf4e97.

Closes: #765366

apt-pkg/deb/dpkgpm.cc

@@ -1545,9 +1545,6 @@ bool pkgDPkgPM::Go(APT::Progress::PackageManager *progress)
 	       _exit(100);
 	 }
 
-	 /* No Job Control Stop Env is a magic dpkg var that prevents it
-	    from using sigstop */
-	 putenv((char *)"DPKG_NO_TSTP=yes");
 	 execvp(Args[0], (char**) &Args[0]);
 	 cerr << "Could not exec dpkg!" << endl;
 	 _exit(100);

doc/dpkg-tech.dbk

@@ -404,15 +404,7 @@ As yet unwritten. You can refer to the other manuals for now. See
 <itemizedlist>
 <listitem>
 <para>
-DPKG_NO_TSTP - if set to a non-null value, this variable causes dpkg to run a
-child shell process instead of sending itself a SIGTSTP, when the user selects
-to background the dpkg process when it asks about conffiles.
-</para>
-</listitem>
-<listitem>
-<para>
-SHELL - used to determine which shell to run in the case when DPKG_NO_TSTP
-is set.
+SHELL - used to determine which shell to run.
 </para>
 </listitem>
 <listitem>