fuzz: do not use inherits in Cargo.toml

This fixes the oss-fuzz build.

Specifically, the build log[1] showed this error:

    Step #3 - "compile-libfuzzer-address-x86_64": error: inherits must
    not be specified in root profile dev

So we just remove it and inline the settings.

PR #817

[1] - https://oss-fuzz-build-logs.storage.googleapis.com/log-c9b61873-8950-4a50-a729-820d5617ff7a.txt

fuzz/Cargo.toml

@@ -29,7 +29,9 @@ opt-level = 3
 debug = true
 
 [profile.dev]
-inherits = "release"
+opt-level = 3
+debug = true
 
 [profile.test]
-inherits = "release"
+opt-level = 3
+debug = true