Do not enable fibers in tracing-tracy

Fibers introduce a significant overhead to each invocation of Tracy
instrumentation calls, which in turn makes all tracing instrumentation
points (with callstack collection disabled) 2x slower than they could
be.

I anticipate that `fibers` still may become unconditional requirement in
the future due to #35, but there's no reason to impose that overhead on
users today.

tracing-tracy/Cargo.toml

@@ -19,7 +19,7 @@ bench = true
 [dependencies]
 tracing-core = { version = "0.1", default-features = false, features = ["std"] }
 tracing-subscriber = { version = "0.3", default-features = false, features = ["fmt", "registry"] }
-client = { package = "tracy-client", path = "../tracy-client", version = "0.13.0", default-features = false, features = ["fibers"] }
+client = { package = "tracy-client", path = "../tracy-client", version = "0.13.0", default-features = false }
 
 [dev-dependencies]
 tracing = { version = "0.1", default-features = false, features = ["std"] }