iter36: remove env-gated DIAG probes (iter29/30/33/35)

Cleans up the campaign's exploratory env-gated dumps now that all
bugs are fixed:
- iter29 LIBVA_HEVC_DUMP_SLICE_TAIL (h265.c) — refuted 40-byte inflation theory
- iter30 LIBVA_TS_SCALE (picture.c) — refuted timestamp magnitude theory
- iter33 LIBVA_VP8_DUMP_FRAME (vp8.c) — led to α-30 fix
- iter35 LIBVA_MPEG2_DUMP_FRAME (mpeg2.c) — confirmed MPEG-2 ctrls correct

Total: -131 lines / +7 lines (α-7 comment refresh).

Preexisting framework env knobs retained:
- LIBVA_V4L2_DUMP_OUTPUT (picture.c α-16)
- LIBVA_V4L2_DUMP_CAPTURE (surface.c)
- LIBVA_V4L2_ZERO_CAPTURE (picture.c)
- LIBVA_V4L2_REQUEST_VIDEO_PATH / MEDIA_PATH / NO_AUTODETECT (request.c)

The 3 load-bearing fixes remain unchanged:
α-25 (rkvdec image_fmt pre-seed, src/context.c)
α-29 (slice_params.short_term_ref_pic_set_size, src/h265.c)
α-30 (VP8 OUTPUT header prepend, src/picture.c)

src/h265.c

@@ -70,7 +70,6 @@
 #include "surface.h"
 
 #include <assert.h>
-#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -631,26 +630,6 @@ int h265_set_controls(struct request_data *driver_data,
 				       cumulative_offset,
 				       &slice_params_array[i]);
 
-		/* iter29 DIAG: dump trailing 80 bytes of each HEVC slice.
-		 * Set LIBVA_HEVC_DUMP_SLICE_TAIL=1 to enable. Goal: characterise
-		 * the 40-byte inflation in ffmpeg-vaapi vs ffmpeg-v4l2request for
-		 * frame 2+ slices (see iter27/28 close). */
-		if (getenv("LIBVA_HEVC_DUMP_SLICE_TAIL")) {
-			uint32_t sz = slice->slice_data_size;
-			uint32_t boff = slice->slice_data_byte_offset;
-			uint8_t *p = (uint8_t *)surface_object->source_data + cumulative_offset;
-			uint32_t dump_n = sz < 80 ? sz : 80;
-			uint32_t start = sz - dump_n;
-			uint32_t k;
-			fprintf(stderr, "iter29 slice[%u] nut=%u size=%u boff=%u start_in_slice=%u tail80:",
-				i, slice_params_array[i].nal_unit_type, sz, boff, start);
-			for (k = 0; k < dump_n; k++) {
-				if ((k & 0xf) == 0) fprintf(stderr, "\n  +%04x:", start + k);
-				fprintf(stderr, " %02x", p[start + k]);
-			}
-			fprintf(stderr, "\n");
-		}
-
 		cumulative_offset += slice->slice_data_size;
 	}