gh-153304: Remove incorrect assertion in pegen error handling

[BHUVANSH855]

Summary

Remove an assertion in _PyPegen_raise_error_known_location() that can abort debug builds before the existing fallback logic handles the error.

When _PyErr_ProgramDecodedTextObject() cannot reopen the source file, it may return NULL. The code immediately following the assertion already handles this case, so the assertion prevents the intended fallback from executing.

Testing

• Reproduced the assertion failure on a Linux --with-pydebug build using the reproducer from the issue.
• Verified that after removing the assertion, the reproducer reports a normal SyntaxError instead of aborting.
• Built CPython successfully on both Linux (--with-pydebug) and Windows Debug builds.

• Issue: Assertion error in _PyPegen_raise_error_known_location() when running PyRun_SimpleFileExFlags() #153304

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