Remove all deprecated codes

[habara-k]

Motivation

With LineBotClient / AsyncLineBotClient established as the single entry point and all legacy code already marked @deprecated in the previous PR, the v2 linebot modules are no longer needed. Removing them reduces the package size, eliminates confusion between two parallel APIs, and ensures new users only see the supported path.

What's changed

Deleted

• linebot/api.py, linebot/async_api.py — legacy LineBotApi / AsyncLineBotApi classes
• linebot/models/ (26 files) — all v2 model classes (Action, FlexMessage, RichMenu, Template, etc.)
• linebot/constants/ — v2 constants
• linebot/http_client.py, linebot/async_http_client.py, linebot/aiohttp_async_http_client.py — v2 HTTP client layer
• linebot/exceptions.py — v2 BaseError, LineBotApiError
• linebot/utils.py, linebot/webhook.py — v2 utilities and webhook (v3 has its own at linebot/v3/webhook.py)
• linebot/deprecations.py — deprecation warning classes (no longer needed)
• tests/api/, tests/async_api/, tests/models/, tests/test_*.py, tests/text/ — all v2 tests
• docs/source/linebot.rst, docs/source/linebot.models.rst — v2 Sphinx docs

Modified

• linebot/__init__.py — stripped down to only export __version__
• setup.py — removed CodegenCommand (generated async_api.py from api.py) and subprocess import
• requirements.txt — removed Deprecated package dependency
• README.rst — replaced "How to suppress deprecation warnings" section with a note that v2 modules have been removed
• docs/source/index.rst — removed linebot from toctree (only linebot.v3 remains)