all: remove more mentions of the vendor folder

[a-nogikh]

The one remaining mention is this:

syzkaller/executor/_include/flatbuffers/README.md

Lines 1 to 7 in af620dc

	This was manually vendored on the version v2.0.8
	(which matches the compiler version in the env container)
	using the following commands:
	git clone --branch=v2.0.8 --depth=1 --single-branch https://github.com/google/flatbuffers.git
	cp flatbuffers/LICENSE.txt syzkaller/vendor/flatbuffers/
	cp flatbuffers/include/flatbuffers/*.h syzkaller/vendor/flatbuffers/

Is that still relevant?

[dvyukov]

The one remaining mention is this:

syzkaller/executor/_include/flatbuffers/README.md

Lines 1 to 7 in af620dc

	This was manually vendored on the version v2.0.8
	(which matches the compiler version in the env container)
	using the following commands:
	git clone --branch=v2.0.8 --depth=1 --single-branch https://github.com/google/flatbuffers.git
	cp flatbuffers/LICENSE.txt syzkaller/vendor/flatbuffers/
	cp flatbuffers/include/flatbuffers/*.h syzkaller/vendor/flatbuffers/

Is that still relevant?

It's left-over from 9d9141e

We need to update the comment rather than remove it.

[a-nogikh]

Cc @tarasmadan

[a-nogikh]

Updated the README. PTAL.