gdbus-codegen: Drop DocBook warning tags from generated code

gtk-doc switched from DocBook to Markdown ages ago. There is no Markdown
equivalent for `<warning>`, so just drop it. It wasn’t adding anything
particularly valuable to the documentation.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1130
Status Job ID Name Coverage
  Style Check
passed #518122
allowed to fail
style-check-diff

00:01:04

 
  Build
passed #518126
cross-android_api21_arm64

00:02:51

passed #518127
cross-android_api28_arm64

00:01:25

passed #518128
cross-mingw64

00:02:01

passed #518124
debian-stable-x86_64

00:02:12

passed #518123
fedora-x86_64

00:09:56

passed #518125
G_DISABLE_ASSERT

00:02:12

passed #518129
win32
msys2-mingw32

00:07:46

passed #518130
win32
vs2017-x64

00:07:54

 
  Coverage
passed #518131
coverage

00:01:11

77.9%
 
  Analysis
passed #518133
scan-build

00:05:01

failed #518132
allowed to fail
valgrind

00:00:44

 
Name Stage Failure
failed
valgrind Analysis
Uploading artifacts...
_build/config.h: found 1 matching files
_build/glib/glibconfig.h: found 1 matching files

_build/meson-logs: found 5 matching files

Uploading artifacts to coordinator... ok
id=518132 responseStatus=201 Created token=BrsiX5pr
Uploading artifacts...
WARNING: _build/valgrind-report.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1