Remove rST-specific code in snippets_test.py

[mhucka]

A regex in this file (on line 143) triggered a code scan security warning:(https://github.com/quantumlib/Cirq/security/code-scanning/97):

def find_rst_code_snippets(content: str) -> List[Tuple[str, int]]:
    snippets = find_code_snippets(
        r'\n.. code-block:: python\n(?:\s+:.*?\n)*\n(.*?)(?:\n\S|\Z)', content
                                           ^^^

This part of the regular expression may cause exponential
backtracking on strings starting with '\naa code-block:: python\n :'
and containing many repetitions of '\n :'.

This was in code for testing reStructuredText files. Since we no longer have any .rst files in the code base, it seemed more sensible to remove all the rST-specific code in here. Doing that not only resolves the code scan warning – it also reduces the maintenance burden going forward.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.17%. Comparing base (a2bf6e8) to head (d47b845).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7085      +/-   ##
==========================================
- Coverage   98.18%   98.17%   -0.01%     
==========================================
  Files        1089     1089              
  Lines       95237    95210      -27     
==========================================
- Hits        93508    93474      -34     
- Misses       1729     1736       +7     

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[pavoljuhas]

Thanks!