gh-117755: Remove tests on huge memory allocations (#117938)

Remove unreliable tests on huge memory allocations:

* Remove test_maxcontext_exact_arith() of test_decimal.
  Stefan Krah, test author, agreed on removing the test:
  https://github.com/python/cpython/issues/114331#issuecomment-1925731273
* Remove test_constructor() tests of test_io.
  Sam Gross suggests remove them:
  https://github.com/python/cpython/pull/117809#pullrequestreview-2003889558

On Linux, depending how overcommit is configured, especially on Linux
s390x, a huge memory allocation (half or more of the full address
space) can succeed, but then the process will eat the full system
swap and make the system slower and slower until the whole system
becomes unusable.

Moreover, these tests had to be skipped when Python is built with
sanitizers.