Remove spec interpreter fuzz target temporarily (#3399)

This commit removes the `differential_spec` fuzz target for now,
although this removal is intended to be temporary. We have #3251 to
track re-enabling the spec interpreter in a way that it won't time out,
and additionally the spec interpreter is also failing to build with
ocaml on oss-fuzz so that will also need to be investigated when
re-enabling.

fuzz/Cargo.toml

@@ -52,12 +52,6 @@ path = "fuzz_targets/differential.rs"
 test = false
 doc = false
 
-[[bin]]
-name = "differential_spec"
-path = "fuzz_targets/differential_spec.rs"
-test = false
-doc = false
-
 [[bin]]
 name = "differential_wasmi"
 path = "fuzz_targets/differential_wasmi.rs"