Remove the libsnapshot fuzzer.

The only bugs in the corpus here were bugs in the fuzzer itself. Part
of the reason is its speed - it's limited by the dm layer so can't fuzz
optimally. That also makes it difficult to maintain, since it's
effectively a functional OTA fuzzer, and is very sensitive to the
implementation of libsnapshot.

After discussing this through with the team, we think the best course is
to sunset this and aim for improved unit and functional OTA testing.

Bug: 227656961
Test: builds
Change-Id: I2ecf838be336ee06459988ef282f0694cebfce51