Remove extra if/then/else construct in pip element

This was introduced in [0] but we can include it in the existing elif
series instead.

[0] I2b75afd310f009ae8614f6ca75bb984b56d25c45

Change-Id: Ibe05f367be997efbd8c5ebec77503ebd9cda1c8b

diskimage_builder/elements/pip-and-virtualenv/install.d/pip-and-virtualenv-source-install/04-install-pip

@@ -250,8 +250,7 @@ EOF
 elif [[ $DISTRO_NAME = gentoo ]]; then
     packages="dev-python/pip dev-python/virtualenv"
     emerge -U $packages
-else
-    if [[ $DIB_RELEASE = bullseye ]]; then
+elif [[ $DIB_RELEASE = bullseye ]]; then
     packages="python3-pip python3-virtualenv"
     apt-get -y install $packages
     install_python3_pip
@@ -275,4 +274,3 @@ else
         install_python3_pip
     fi
 fi
-fi