Don't throw from firebird_rowid_backend destructor.

This is useless as the exception would already be thrown if an object of this
class were constructed, there is no need to also throw it when destroying it
and doing this results in compilation warnings from at least MSVC.

Signed-off-by: Vadim Zeitlin <vz-soci@zeitlins.org>

src/backends/firebird/row-id.cpp

@@ -18,6 +18,4 @@ firebird_rowid_backend::firebird_rowid_backend(firebird_session_backend & /* ses
 
 firebird_rowid_backend::~firebird_rowid_backend()
 {
-    // Unsupported in Firebird backend
-    throw soci_error("RowIDs are not supported");
 }