linux-image: Stop setting $STEM in environment for maintainer script hooks

This seems to have been a transitional feature for the switch
from kernel-image to linux-image.  mkvmlinuz still uses it but
no hook scripts appear to depend on it.

debian/changelog

@@ -20,6 +20,7 @@ linux (4.6-1~exp2) UNRELEASED; urgency=medium
   * [mips{,64}r6{,el}] Add kernel flavours for the Malta platform; build
     linux-libc-dev and tools packages (Closes: #825024; thanks to YunQiang Su)
   * [mips*/*-malta] udeb: Add ata-modules package
+  * linux-image: Stop setting $STEM in environment for maintainer script hooks
 
   [ Aurelien Jarno ]
   * [mips64{,el}] Set CPU to MIPS64 R2.

debian/templates/image.postinst.in

@@ -496,9 +496,6 @@ else {
   }
 }
 
-# set the env var stem
-$ENV{'STEM'} = "linux";
-
 sub run_hook {
   my $type   = shift;
   my $script = shift;

debian/templates/image.postrm.in

@@ -144,9 +144,6 @@ sub image_magic {
   }
 }
 
-# set the env var stem
-$ENV{'STEM'} = "linux";
-
 sub system_failure_message {
   if ($? < 0) {
     return "$!";

debian/templates/image.preinst.in

@@ -36,9 +36,6 @@ if (-r "$CONF_LOC" && -f "$CONF_LOC"  ) {
   }
 }
 
-# set the env var stem
-$ENV{'STEM'} = "linux";
-
 sub system_failure_message {
   if ($? < 0) {
     return "$!";

debian/templates/image.prerm.in

@@ -81,9 +81,6 @@ if ($running eq $version) {
 
 chdir("/") or die "could not chdir to /:$!\n";
 
-# set the env var stem
-$ENV{'STEM'} = "linux";
-
 sub system_failure_message {
   if ($? < 0) {
     return "$!";