chore(tvix/eval): prune some dependencies & features

* We no longer need backtrace-on-stack-overflow, as we no longer
  overflow the stack with the recent eval refactorings. This was weird
  voodoo anyways, introduced earlier to debug some cases where stack
  overflows occured.

* default features of genawaiter crate are not needed, as we don't use
  their proc macros

Change-Id: I346fc5a18d7f117ee805909a8be8f535b96be76c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8263
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/eval/src/vm/mod.rs

@@ -229,15 +229,6 @@ impl<'o> VM<'o> {
         globals: Rc<GlobalsMap>,
         reasonable_span: LightSpan,
     ) -> Self {
-        // Backtrace-on-stack-overflow is some seriously weird voodoo and
-        // very unsafe.  This double-guard prevents it from accidentally
-        // being enabled on release builds.
-        #[cfg(debug_assertions)]
-        #[cfg(feature = "backtrace_overflow")]
-        unsafe {
-            backtrace_on_stack_overflow::enable();
-        };
-
         Self {
             nix_search_path,
             io_handle,