Remove debug artifacts from modeling_apriel2

Remove all debug-only code that accumulated during development:
- DEBUG_* flag definitions (DEBUG_GDN_LAYER, DEBUG_GDN_STATE, DEBUG_GDN_OUTPUT,
  DEBUG_KDA_LAYER, DEBUG_DECODER_LAYER, DEBUG_FINAL_NORM, DEBUG_LM_HEAD, DEBUG_SYNC)
- _debug_print, _debug_tensor, _debug_state_stats helper methods
- All if DEBUG_*: blocks with print statements
- All commented-out # self._debug_* / # self._cached_* lines
- SYNC-1..6 CUDA sync debug points
- import os (only used by DEBUG_SYNC)

Functional code and bug fixes from previous commits are unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: oleksost