leaflet: Don't implement GtkBuildableIface

The implementation is just a useless empty shell.

Fixes Librem5/libhandy#197.
11 jobs for leaflet-buildable in 21 minutes and 27 seconds (queued for 30 minutes and 45 seconds)
Status Job ID Name Coverage
  Build
passed #246485
librem5
api-visibility

00:02:24

passed #246487
librem5
build-debian-clang

00:03:21

passed #246486
librem5
build-debian-gcc

00:02:42

passed #246488
librem5
build-fedora-gcc

00:02:41

 
  Test+Docs
passed #246489
librem5
build-gtkdoc

00:02:33

passed #246490
librem5
unit-test

00:02:33

43.6%
 
  Package
passed #246491
librem5
package-debian-buster

00:02:27

passed #246492
librem5:arm64
package-debian-buster:arm64

00:06:54

failed #246493
flatpak allowed to fail
package-flatpak

00:00:53

 
  Test Package
passed #246494
librem5
autopkgtest-debian-buster-package

00:02:31

passed #246495
librem5
lintian-debian-buster-package

00:00:58

 
Name Stage Failure
failed
package-flatpak Package
Committing stage finish to cache
Exporting sm.puri.Handy.Demo to repo
error: Writing content object: min-free-space-percent '3%' would be exceeded, at least 4.1?kB requested
Export failed: Child process exited with code 1
Uploading artifacts...
_build/meson-logs/meson-log.txt: found 1 matching files
WARNING: sm.puri.Handy.Demo-dev.flatpak: no matching files
Uploading artifacts to coordinator... ok
id=246493 responseStatus=201 Created token=L-Zk5RJ7
ERROR: Job failed: exit code 1