remove slow textencoder branch

[anonrig]

[DO NOT MERGE]

Blocked until fast-text-encoder autogate is rolled out.

[ask-bonk]

The change is straightforward — it removes the slow fallback branches guarded by the ENABLE_FAST_TEXTENCODER autogate in both encode() and encodeInto(), making the fast simdutf-based implementation the only code path. The #include <workerd/util/autogate.h> is also correctly removed since it's no longer needed.

One issue I notice: after this PR, the ENABLE_FAST_TEXTENCODER autogate key definition in autogate.h and autogate.c++ becomes dead code — it has no remaining consumers. This should be cleaned up in this PR or a follow-up.

The PR description says "Blocked until fast-text-encoder autogate is rolled out", which is the correct approach — the autogate must be fully rolled out before removing the fallback.

LGTM

github run

[codspeed-hq]

Merging this PR will improve performance by ×8.7

⚡ 16 improved benchmarks
✅ 54 untouched benchmarks
⏩ 129 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	Encode_ASCII_1024[TextEncoder][0/0/1024]	3.7 ms	3.2 ms	+15.65%
⚡	EncodeInto_TwoByte_256[TextEncoder][1/2/256]	5.3 ms	2.8 ms	+88.42%
⚡	EncodeInto_TwoByte_8192[TextEncoder][1/2/8192]	96.6 ms	11.1 ms	×8.7
⚡	Encode_ASCII_256[TextEncoder][0/0/256]	3.1 ms	2.6 ms	+20.57%
⚡	Encode_OneByte_256[TextEncoder][0/1/256]	5.2 ms	3 ms	+74.33%
⚡	Encode_ASCII_8192[TextEncoder][0/0/8192]	13.5 ms	8.6 ms	+57.26%
⚡	Encode_OneByte_1024[TextEncoder][0/1/1024]	12 ms	4.5 ms	×2.7
⚡	Encode_OneByte_8192[TextEncoder][0/1/8192]	84.7 ms	18.5 ms	×4.6
⚡	Encode_TwoByte_1024[TextEncoder][0/2/1024]	16.4 ms	5.1 ms	×3.2
⚡	Encode_TwoByte_8192[TextEncoder][0/2/8192]	119.5 ms	23.3 ms	×5.1
⚡	Encode_ASCII_32[TextEncoder][0/0/32]	3.2 ms	2.4 ms	+32.57%
⚡	Encode_TwoByte_256[TextEncoder][0/2/256]	6.5 ms	3.3 ms	+97.22%
⚡	EncodeInto_TwoByte_1024[TextEncoder][1/2/1024]	14.1 ms	3.5 ms	×4
⚡	EncodeInto_OneByte_1024[TextEncoder][1/1/1024]	10.4 ms	3.4 ms	×3
⚡	EncodeInto_OneByte_8192[TextEncoder][1/1/8192]	66.5 ms	9.6 ms	×7
⚡	EncodeInto_OneByte_256[TextEncoder][1/1/256]	4.4 ms	2.8 ms	+57.09%

---

_{Comparing yagiz/remove-slow-text-encoder (82f0c46) with main (755795d)}

Footnotes

1. 129 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩