Update perl-DBD-ODBC.spec

1. Remove old lines for RHEL5 only.(BuildRoot, %clean section, defattr)

2. We can drop the explicit version requires as they are really old, even RHEL5 has shipped higher version than the minimum.

3. I dropped some README files, as we are Linux only.

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This is just a small change from the aspect of SPEC only, there might have other issues, which can be found later in fedora-review.