Erase GI annotation: skip g_cancellable_source_new

Deletes the skip annotation from g_cancellable_source_new(). This was
originally added because GSource wasn't introspectable, but this is no
longer an issue as G_TYPE_SOURCE was added in 2.30.

Fixes: #1877
12 jobs for master in 28 minutes and 40 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #401127
cross-android_api21_arm64

00:02:46

passed #401128
cross-android_api28_arm64

00:02:56

passed #401129
cross-mingw64

00:03:42

passed #401125
debian-stable-x86_64

00:02:52

passed #401124
fedora-x86_64

00:05:05

passed #401126
G_DISABLE_ASSERT

00:02:48

passed #401130
win32
msys2-mingw32

00:13:54

passed #401131
win32
vs2017-x64

00:10:05

 
  Coverage
passed #401132
coverage

00:01:22

77.9%
 
  Analysis
passed #401133
scan-build

00:12:13

 
  Deploy
passed #401134
pages

00:01:09

passed pages:deploy

00:00:03