fate: Fix dependencies for ffprobe tests with external samples

fate: Fix dependencies for ffprobe tests with external samples

Up until now, they were appended to the FATE_EXTERN-$(CONFIG_FFMPEG)
variable and were therefore activated when ffmpeg was enabled regardless
of whether ffprobe was enabled.

Also the same happened with FATE_SAMPLES_FASTSTART, although the
corresponding test (mov-faststart-4gb-overflow) only requires external
samples.

Furthermore, remove the unused FATE_FULL variable (FATE_EXTERN_FFPROBE has
taken its place).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>