Revert "Now lambda is counted as a valid context in …"

[JukkaL]

This reverts commit d37c2be.

Lambdas can't be safely deferred. There are various subtle assumptions about
lambdas not being deferred in different parts of the codebase, and figuring out
how to do this safely is almost certainly not worth the trouble. This would be
very difficult to validate, since lambda deferral would probably happen very
rarely in practice.

Also, I've seen some crashes which might be caused by this (not sure though).

[JukkaL]

cc @sobolevn

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

graphql-core (https://github.com/graphql-python/graphql-core)
- tests/execution/test_union_interface.py:69: error: Unused "type: ignore" comment
- tests/execution/test_union_interface.py:75: error: Unused "type: ignore" comment
- tests/execution/test_union_interface.py:76: error: Unused "type: ignore" comment
- tests/execution/test_union_interface.py:77: error: Unused "type: ignore" comment
- tests/execution/test_union_interface.py:87: error: Unused "type: ignore" comment
- tests/execution/test_union_interface.py:88: error: Unused "type: ignore" comment
- tests/execution/test_union_interface.py:89: error: Unused "type: ignore" comment
- tests/execution/test_union_interface.py:100: error: Unused "type: ignore" comment
- tests/execution/test_union_interface.py:101: error: Unused "type: ignore" comment
- tests/execution/test_union_interface.py:102: error: Unused "type: ignore" comment
- tests/execution/test_union_interface.py:127: error: Unused "type: ignore" comment
- tests/execution/test_union_interface.py:128: error: Unused "type: ignore" comment
- tests/execution/test_union_interface.py:129: error: Unused "type: ignore" comment

[JukkaL]

Also, I've seen some crashes which might be caused by this (not sure though).

I checked that the revert fixed this crash I was experiencing:

Traceback (most recent call last):
  File "/usr/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "mypy/checkexpr.py", line 3942, in accept
  File "mypy/nodes.py", line 2054, in accept
  File "mypy/checkexpr.py", line 3753, in visit_list_comprehension
  File "mypy/checkexpr.py", line 3780, in check_generator_or_comprehension
  File "/usr/lib/python3.8/contextlib.py", line 113, in __enter__
    return next(self.gen)
  File "mypy/binder.py", line 402, in frame_context
AssertionError: