picture_pool: test: remove picture_pool_GetSize call

There's already a test to check that we can get as much pictures as we
initialized the pool with.
16 jobs for !25 with picture-clean/2 in 84 minutes and 56 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Build
passed #421623
amd64 docker
android-arm

00:05:26

passed #421624
amd64 docker
android-arm64

00:06:05

passed #421625
amd64 docker
android-x86

00:04:50

passed #421626
amd64 docker
android-x86_64

00:04:49

passed #421615
amd64 docker
debian

00:05:35

passed #421619
macos
ios-arm64

00:14:57

passed #421620
macos
ios-armv7

00:13:15

passed #421621
macos
ios-simulator-x86_64

00:13:36

passed #421618
macos
macos

00:16:24

passed #421617
amd64 docker
raspbian-arm

00:07:09

passed #421616
amd64 docker
snap

00:12:52

passed #421622
macos
tvos-arm64

00:13:49

passed #421614
amd64 docker
uwp64-libvlc-llvm

00:04:25

passed #421611
amd64 docker
win32

00:03:05

passed #421612
amd64 docker
win64

00:06:42

passed #421613
amd64 docker
win64-llvm

00:04:40