Remove unused exception parameter from gloo/rendezvous/context.cc

[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.

Reviewed By: palmje

Differential Revision: D51785901

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D51785901