installer: drop extraneous setting of VERSION_CODENAME in mantic

It already has this value.
The reason jammy_install has it is that it's reused for Mint.

installer/v3.sh

@@ -54,7 +54,6 @@ mantic_install()
     apt-get update
     apt -y install gpg software-properties-common wget
     wget -q https://dl.bluecherrydvr.com/key/bluecherry.asc -O- | sudo tee /etc/apt/trusted.gpg.d/bluecherry.asc
-    VERSION_CODENAME=mantic
     : "${SRCLIST_URL:=https://dl.bluecherrydvr.com/sources.list.d/bluecherry-"$VERSION_CODENAME"-unstable.list}"
     wget --output-document=/etc/apt/sources.list.d/bluecherry-"$VERSION_CODENAME".list "$SRCLIST_URL"
     apt -y update