Remove unused exception parameter from multifeed/recommendation_platform/corpus/backfill/BackfillMain.cpp

[r-barnes]

Summary:
-Wunused-exception-parameter has identified an unused exception parameter. This diff removes it.

This:

try {
    ...
} catch (exception& e) {
    // no use of e
}

should instead be written as

} catch (exception&) {

If the code compiles, this is safe to land.

Differential Revision: D88448341

[pytorch-bot]

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[meta-codesync]

@r-barnes has exported this pull request. If you are a Meta employee, you can view the originating Diff in D88448341.