Remove special handling of kw_only in dataclass plugin serialization

[sobolevn]

It is not needed, because it is always serialized as bool and is always present.

It was added a long time ago in #10867 as a compat layer. But, since then we've added at least one is_neither_frozen_nor_nonfrozen attribute that is serialized / deserialized in a common way.

So, let's remove this hack for good.

[sobolevn]

@ikonst do you want to take a look?

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[ikonst]

Sounds good. Curious if it was ever needed. It looks like cache is invalidated when version_id changes, and version_id contains the current git rev:

mypy/mypy/version.py

Lines 15 to 18 in 5b6dc5f

	if __version__.endswith("+dev") and git.is_git_repo(mypy_dir) and git.have_git():
	__version__ += "." + git.git_revision(mypy_dir).decode("utf-8")
	if git.is_dirty(mypy_dir):
	__version__ += ".dirty"