Omit redundant File block in SyntaxError output when crash line has location

[Dextheking1]

Stacked on #14899 (which makes the crash line carry the error's own file:line:col); closes #14994.

Before

E     File "package/lib.py", line 1
E       def broken(:
E                  ^
E   SyntaxError: invalid syntax
package/lib.py:1:12: SyntaxError: invalid syntax

After

E   SyntaxError: invalid syntax
package/lib.py:1:12: SyntaxError: invalid syntax

Rationale

traceback.format_exception_only() emits a File "...", line N / source / caret block for SyntaxError, which ExceptionInfoFormatter.get_exconly() renders as E-lines. Once the crash line reports the error's own file:line:column (#14899), that block only repeats the same location, so it is now dropped in get_exconly() via a small _strip_syntax_error_file_block() helper (keeps the exception line itself verbatim; never strips when the exception line can't be identified).

When the error does not carry usable location info (no filename/lineno/offset), the crash line falls back to the traceback entry and the block is kept, so no information is ever lost:

E     File "weird.py", line 3
E       src
E   SyntaxError: custom import error

• Include new tests or update existing tests when applicable.
• Create a new changelog file in the changelog directory (changelog/14994.improvement.rst).
• Allow maintainers to push and squash when merging my commits.
• Closes Redundant File ..., line N block in SyntaxError output #14994
• If AI agents were used, they are credited: AI assistance was used in preparing this change (as in Fix SyntaxError crash line to show file, line, and column #14899).

Closes #14994.