chore(tvix): drop json feature for tracing-subscriber

Drop the (unused) feature flags here, and get rid of some crate
dependencies.

Change-Id: Id64852b498725467d56abb12b548301cfba6a760
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11678
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/Cargo.lock

@@ -4009,16 +4009,6 @@ dependencies = [
  "web-time",
 ]
 
-[[package]]
-name = "tracing-serde"
-version = "0.1.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bc6b213177105856957181934e4920de57730fc69bf42c37ee5bb664d406d9e1"
-dependencies = [
- "serde",
- "tracing-core",
-]
-
 [[package]]
 name = "tracing-subscriber"
 version = "0.3.18"
@@ -4029,15 +4019,12 @@ dependencies = [
  "nu-ansi-term",
  "once_cell",
  "regex",
- "serde",
- "serde_json",
  "sharded-slab",
  "smallvec",
  "thread_local",
  "tracing",
  "tracing-core",
  "tracing-log",
- "tracing-serde",
 ]
 
 [[package]]