tests: Delete old GSlice tests

Since the implementation of GSlice has been deleted, we don’t need to
test it any more.

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

Helps: #1079
11 jobs for 1079-drop-gslice in 33 minutes and 16 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #333549
cross-android_api21_arm64

00:01:14

passed #333550
cross-android_api28_arm64

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passed #333551
cross-mingw64

00:04:22

passed #333569
debian-stable-x86_64

00:01:54

passed #333546
fedora-x86_64

00:07:10

passed #333548
G_DISABLE_ASSERT

00:04:42

passed #333552
win32
msys2-mingw32

00:08:36

passed #333553
win32
vs2017-x64

00:09:14

failed #333547
debian-stable-x86_64

00:04:37

 
  Coverage
passed #333554
coverage

00:01:23

78.0%
 
  Analysis
passed #333555
scan-build

00:09:47