Remove legacy panic handling (#37947)

@maxdeviant We can eventually turn down the panic telemetry endpoint,
but should probably leave it up while there's still a bunch of stable
users hitting it.

@maxbrunsfeld We're optimistic that this change also fixed the macos
crashed-thread misreporting. We think it was because the
`CrashContext::exception` was getting set to `None` only on macos, while
on linux it was getting a real exception value from the sigtrap. Now
we've unified and it uses `SIGABRT` on both platforms (I need to double
check that this works as expected for windows).

We unconditionally set `RUST_BACKTRACE=1` for the current process so
that we see backtraces when running in a terminal by default. This
should be fine but I just wanted to note it since it's a bit abnormal.

Release Notes:

- N/A

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

Cargo.lock

@@ -2853,9 +2853,9 @@ dependencies = [
 
 [[package]]
 name = "cfg-if"
-version = "1.0.0"
+version = "1.0.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
+checksum = "2fd1289c04a9ea8cb22300a459a72a385d7c73d3259e2ed7dcb2af674838cfa9"
 
 [[package]]
 name = "cfg_aliases"
@@ -4040,6 +4040,7 @@ name = "crashes"
 version = "0.1.0"
 dependencies = [
  "bincode",
+ "cfg-if",
  "crash-handler",
  "log",
  "mach2 0.5.0",
@@ -13502,9 +13503,7 @@ dependencies = [
  "askpass",
  "assistant_tool",
  "assistant_tools",
- "backtrace",
  "cargo_toml",
- "chrono",
  "clap",
  "client",
  "clock",
@@ -13549,7 +13548,6 @@ dependencies = [
  "shellexpand 2.1.2",
  "smol",
  "sysinfo",
- "telemetry_events",
  "thiserror 2.0.12",
  "toml 0.8.20",
  "unindent",
@@ -20512,7 +20510,6 @@ dependencies = [
  "breadcrumbs",
  "call",
  "channel",
- "chrono",
  "clap",
  "cli",
  "client",
@@ -20563,7 +20560,6 @@ dependencies = [
  "language_selector",
  "language_tools",
  "languages",
- "libc",
  "line_ending_selector",
  "log",
  "markdown",