collapse ubuntu mapping for libfltk-dev

All releases of Ubuntu since xenial use the same package name for
[`libfltk-dev`](https://packages.ubuntu.com/jammy/libfltk1.3-dev),
therefore we can collapse the mapping.
This has the added benefit that this key will also work on `jammy` and
`kinetic`.

rosdep/base.yaml

@@ -3115,25 +3115,7 @@ libfltk-dev:
   opensuse: [fltk-devel]
   rhel: [fltk-devel]
   slackware: [fltk]
-  ubuntu:
-    artful: [libfltk1.3-dev]
-    bionic: [libfltk1.3-dev]
-    focal: [libfltk1.3-dev]
-    lucid: [libfltk1.1-dev]
-    maverick: [libfltk1.1-dev]
-    natty: [libfltk1.1-dev]
-    oneiric: [libfltk1.1-dev]
-    precise: [libfltk1.1-dev]
-    quantal: [libfltk1.1-dev]
-    raring: [libfltk1.1-dev]
-    saucy: [libfltk1.1-dev]
-    trusty: [libfltk1.1-dev]
-    utopic: [libfltk1.3-dev]
-    vivid: [libfltk1.3-dev]
-    wily: [libfltk1.3-dev]
-    xenial: [libfltk1.3-dev]
-    yakkety: [libfltk1.3-dev]
-    zesty: [libfltk1.3-dev]
+  ubuntu: [libfltk1.3-dev]
 libfontconfig1-dev:
   debian: [libfontconfig1-dev]
   fedora: [fontconfig-devel]