chore: Don't use bpf_core_enum_value_exists for detecting bpf_loop

We already have a more reliable `FEATURE_BPF_LOOP` check for that.

Author: vadorovsky

crates/bpf-builder/include/loop.bpf.h

@@ -16,14 +16,7 @@
 // function seems to cause issues.
 #ifdef FEATURE_BPF_LOOP
 #define LOOP(max_iterations, max_unroll, callback_fn, ctx)                     \
-  if (bpf_core_enum_value_exists(enum bpf_func_id, BPF_FUNC_loop)) {           \
-    bpf_loop(max_iterations, callback_fn, ctx, 0);                             \
-  } else {                                                                     \
-    _Pragma("unroll") for (int i = 0; i < max_unroll; i++) {                   \
-      if (callback_fn(i, ctx) == LOOP_STOP)                                    \
-        break;                                                                 \
-    }                                                                          \
-  }
+  bpf_loop(max_iterations, callback_fn, ctx, 0);
 #else
 // On kernel <= 5.13 taking the address of a function results in a verifier
 // error, even if inside a dead-code elimination branch.