fix(httpapi): drop x\b workaround for fixed Claude Code 0.2.70 paste-echo bug

[mzihlmann]

Fixes #223.

The x\b workaround added in b732081 to mask Claude Code 0.2.70's bracketed-paste echo bug now produces a visible stray x at the end of every prompt with current Claude Code (2.1.x). The original commit message promised removal "a couple of days after the new version is released" — almost a year on, the hack is still here and is now actively breaking input (e.g. slash-command argument parsing receives /cmd argx instead of /cmd arg).

The companion removeDuplicateClaude_0_2_70_Output function from the original commit is already gone from the codebase, so this reverts only the lib/httpapi/claude.go portion.

Verified locally against Claude Code 2.1.143: prompts render cleanly without the trailing x, and slash-command arguments parse correctly.

[github-actions]

✅ Preview binaries are ready!

To test with modules: agentapi_version = "agentapi_224" or download from: https://github.com/coder/agentapi/releases/tag/agentapi_224

[35C4n0r]

I can't seem to reproduce the bug mentioned in the PR description, but the hack is also no longer needed.
LGTM!

[mzihlmann]

claude was able to handle the stray x as a typo most of the time, but it became obvious when the prompt ended with something that reasonably could contain an x, an image name in my case: alpine:3.23x. When pushed claude said that every prompt had a stray x appended to it.