fuzzing: Combine the compile and compile-mutate fuzz targets (#4048)

We should still get the same amount of fuzzing using libfuzzer's mutators and
using `wasm-mutate` as a mutator now, but they can share the same corpus,
allowing mutations that one performed but the other didn't to reach new areas.

fuzz/Cargo.toml

@@ -30,12 +30,6 @@ path = "fuzz_targets/compile.rs"
 test = false
 doc = false
 
-[[bin]]
-name = "compile-mutate"
-path = "fuzz_targets/compile-mutate.rs"
-test = false
-doc = false
-
 [[bin]]
 name = "instantiate"
 path = "fuzz_targets/instantiate.rs"