Remove unused exception parameter from cachelib/shm/PosixShmSegment.cpp

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: D51785883

fbshipit-source-id: 2ae16231cc25686c92384dc4bf00191741079de5

cachelib/shm/PosixShmSegment.cpp

@@ -225,7 +225,7 @@ PosixShmSegment::~PosixShmSegment() {
     // delete the reference mapping so the segment can be deleted if its
     // marked to be.
     deleteReferenceMapping();
-  } catch (const std::system_error& e) {
+  } catch (const std::system_error&) {
   }
 
   // need to close the fd without throwing any exceptions. so we call close