dev-manual: Removed #!/bin/sh -e from the base script form

This start to the script is not really necessary and is in fact
the rare case.  I have removed two occurrences of it in the
"Post-Installation Scripts" section where it was used in code
as the best form to create a script.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
(From yocto-docs rev: f7c1a1e135e8a9d76005bb6ae7b9cc2810f0af90)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/dev-manual-common-tasks.xml

@@ -2664,7 +2664,6 @@
                 A post-installation function has the following structure:
                 <literallayout class='monospaced'>
      pkg_postinst_PACKAGENAME() {
-     #!/bin/sh -e
      # Commands to carry out
      }
                 </literallayout>
@@ -2687,7 +2686,6 @@
                 structure in the post-installation script:
                 <literallayout class='monospaced'>
      pkg_postinst_PACKAGENAME() {
-     #!/bin/sh -e
      if [ x"$D" = "x" ]; then
           # Actions to carry out on the device go here
      else